IUPAC naming of double salt
According to the current version of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book), alphabetical order is used.

Alphabetical order is used in formulae as follows.
(a) Within the group of cations and within the group of anions, respectively, in formulae of salts and double salts. Deviations from this rule may be acceptable if it is desired to convey specific structural information (…)

and

Alphabetical order is used in names as follows.
(d) In compositional names, the names of the formally electropositive components and the names of the formally electronegative components are each arranged alphabetically with the former group of components preceding the latter. (…)

Note that the order of components in the name may deviate from the order of the corresponding components in the formula. For example:
$\ce{KMgF3}$ (since K &lt; M)
magnesium potassium fluoride (since m &lt; p)
$\ce{ZnI(OH)}$ (since I &lt; O)
zinc hydroxide iodide (since h &lt; i)