How to write the formula of a mixed metal salt?
&quot;$\ce{KNaSO4}$&quot; is correct, but not because of electronegativity order, but rather the alphabetical one.
From IUPAC &quot;Red Book&quot;, section IR-2.15 ORDERING PRINCIPLES [1, p. 40]:

IR-2.15.2 Alphabetical order
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, as in Example 5 below.
Examples:

$\ce{BiClO}$ (anions $\ce{Cl-}$ and $\ce{O^2-})$
$\ce{NaOCl}$ (the anion is $\ce{OCl-},$ cf. Section IR-4.4.3.1)
$\ce{KNa4Cl(SO4)2}$
$\ce{CaTiO3}$ (perovskite type)
$\ce{SrFeO3}$ (perovskite type)


Reference

IUPAC “Red Book” Nomenclature of Inorganic Chemistry, 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; IUPAC Recommendations; Royal Society of Chemistry: Cambridge, UK, 2005. (PDF)