How to arrive at the preferred IUPAC name for 1,2-diphenylethane?
The essential problem in this case is finding the parent structure. According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), a structure with two or more identical units that are linked by di- or polyvalent groups can be named in two ways:


by ordinary substitutive nomenclature, in which one of the parent structures is chosen as the senior parent structure and the remainder of the structure is expressed by substituent prefixes
by multiplicative nomenclature, in which two or more parent structures are connected by symmetrical or unsymmetrical single or concatenated substituent groups


Both methods yield correct names, which may be used in general nomenclature.

1. Substitutive nomenclature

In many related simple cases, the compound consists of just a ring and a chain so that the parent structure could be the ring or the chain.

The corresponding rule in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:


  P-44.1.2.2 Systems composed of rings and chains (exclusive of linear phanes)
  
  Two methods are recognized to name systems composed of rings and chains
  (exclusive of linear phanes).
  
  (1) Within the same class, a ring or ring system has seniority over a chain. When a ring and a chain contain the same senior element, the ring is chosen as parent. Rings and chains are chosen regardless of their degree of hydrogenation. As a consequence, this approach prefers the choice of a ring over a chain in systems composed of cyclic and acyclic hydrocarbons.
  
  (2) The context may favor the ring or the chain, so that, for example, substituents may be treated alike or an unsaturated acyclic structure may be recognized, or the one chosen has the greater number of skeletal atoms in the ring or in the principal chain of the acyclic structure.
  
  (…) For selection of a preferred IUPAC name, see P-52.2.8.


 


  P-52.2.8 Selection between a ring and a chain as parent hydride
  
  Within the same heteroatom class and for the same number of characteristic groups cited as the principal characteristic group, a ring is always selected as the parent hydride to construct a preferred IUPAC name. In general nomenclature, a ring or a chain can be the parent hydride (see P-44.1.2.2).


These rules apply to compounds that consist of one ring and one chain. The compound that is given in the question, however, consists of a symmetrical chain with two rings. Extending the use of the above-mentioned rules to this case, we could pick one of the rings as the parent structure in accordance with Rule P-52.2.8. This approach would result in the correct name (2-phenylethyl)benzene. Since this method usually yields the preferred IUPAC in many simple cases, it should not be a surprise if some computer-generated names follow this pattern also in this case. Note, however, that the name “2-phenylethylbenzene” suggested in Pubchem is not correct and ambiguous (“2‑phenylethylbenzene” could be interpreted as “(2‑phenylethyl)benzene” or “2‑phenyl‑1‑ethylbenzene”).

In accordance with P-44.1.2.2, the context may favour the chain as parent structure. For the compound that is given in the question, the context may favour the chain in order to highlight the symmetrical shape of the structure, in order to treat the equivalent rings alike, or because a following reaction happens at the chain. This approach would result in the correct name 1,2-diphenylethane.

2. Multiplicative nomenclature

The IUPAC recommendations explain when multiplicative nomenclature is used to generate the preferred IUPAC name (PIN) as follows.


  P-45.1 MULTIPLICATION OF IDENTICAL SENIOR PARENT STRUCTURES
  
  P-45.1.1 Multiplicative nomenclature is senior to substitutive nomenclature for generating preferred IUPAC names to express multiple occurrences of identical senior parent structures, other than alkanes, in the name of the parent structure (see P-51.3.1). In most cases multiplicative names are shorter than regular substitutive names. A preferred IUPAC name is generated by multiplicative nomenclature when the following criteria for its use are met (see P-51.2.3).
  (1) the linking bonds (single or multiple) between the central substituent group of the multiplicative group and all subsequent structural units are identical; and
  (2) the multiplicative groups, other than the central multiplicative group, are symmetrically substituted; and
  (3) the locants of all substituent groups on the identical parent structures, including suffix groups, are identical.
  
  When these conditions are not met, substitutive nomenclature generates preferred IUPAC names.


The compound that is given in the question fulfils these requirements; therefore, the preferred IUPAC name is generated by multiplicative nomenclature as follows.


  P-15.3.1.3 Multiplicative name formation
  Multiplicative names are formed in accordance with the number of occurrences of identical structural units as defined in P-15.3.1.1 and the relationship of the linking multiplicative substituent group to the identical structural units.
  When a compound contains identical structural units as defined in P-15.3.1.1 linked by a symmetrical simple, compound, complex, or concatenated multiplicative group (a di- or polyvalent substituent group), it is named by stating successively:  
  
  (a) the locants for the positions of substitution of the linking multiplicative substituent atom or group to the identical parent structural unit (the locant 1 is omitted when alone in the name of a mononuclear parent hydride);  


Here: 1,1'


  (b) the name of the linking multiplicative substituent atom or group;  


Here: ethane-1,2-diyl


  (c) the numerical prefix ‘di’, ‘tri’, etc.; and/or ‘bis-‘, ‘tris-‘, etc., with no elision of the final vowel before the name of the identical parent structural unit;  


Here: di


  (d) the name of one of the identical structural units including the principal characteristic group and substituents, if any, enclosed in appropriate enclosing marks (see P-16.5).


Here: benzene


  The numbering of the identical parent structural unit is retained and, when there is a choice, the locants of the point of substitution by the linking multiplicative substituent groups on the identical parent structure are as low as possible.


Therefore, the preferred IUPAC name is 1,1'-(ethane-1,2-diyl)dibenzene.