Is it okay to place two battery chargers on one 30 amp circuit?
First things first: ignore the tap conductor rules in the NEC!  You can't use them for receptacles anyway: 240.21(A) refers you to 210.19, which sadly is a bit confusing-looking as it refers to receptacles in two different places, only one of which refers to a notion of 'tap conductor', which furthermore isn't in the NEC's definitions section!  But, 210.19(A)(2):


  Branch Circuits with More than One Receptacle.
  Conductors of branch circuits supplying more than one receptacle for cord-and-
  plug-connected portable loads shall have an ampacity of not less than the 
  rating of the branch circuit.


states that conductors on a multiple receptacle outlet branch circuit must have the same ampacity as the rest of the branch circuit (or better, of course), and Exception 1 to 210.19(A)(4):


  Exception No.1: Tap conductors shall have an ampacity sufficient for the load 
  served. In addition, they shall have an ampacity of not less than 15 for
  circuits rated less than 40 amperes and not less than 20 for circuits rated
  at 40 or 50 amperes and only where these tap conductors supply any of the 
  following loads:
  
  (a) lndividual lamp holders or luminaires with taps extending not longer than 
  450 mm (18 in.) beyond any portion of the lampholder or luminaire.
  
  (b) A luminaire having tap conductors as provided in 410.117.
  
  (c) individual outlets, other than receptacle outlets, with taps not over 450 > mm (18 in.) long.
  
  (d) Infrared lamp industrial heating appliances.
  
  (e) Nonheating leads of deicing and snow-melting cables and mats.


explicitly excludes receptacle outlets.

This leaves you with two options here:


Put a pair of 6-30 receptacles in the garage (yes, you must use 30A), and leave the circuit otherwise as-is, wiring the new receptacle(s) using 10/2.  (Using 15A receptacles on a 30A circuit violates NEC 210.21(B)(3) and the associated table.)
IF you have a 120/240VAC circuit running to the garage (2 hots, neutral, ground), you can fit a subpanel there and then use a pair of dual pole 15A breakers and 14/2 wire to run independent circuits to 6-15R outlets for each charger.  If the existing 30A circuit was run using 10/2 though, you cannot do this -- single pole breakers aren't rated for 240VAC service!