Should equiatomic iron–cobalt alloy be written as FeCo or CoFe?
Current IUPAC recommendations suggest following electronegativity ordering. The symbol for iron (a less electronegative element among the two) should be written first: $\ce{FeCo}.$
From the section IR-4.4 Sequence of citation of symbols in formulae [1, pp. 58–59]:

IR-4.4.2 Ordering principles
IR-4.4.2.1 Electronegativity
If electronegativity is taken as the ordering principle in a formula or a part of a formula, the atomic symbols are cited according to relative electronegativities, the least electronegative element being cited first. For this purpose, Table VI* is used as a guide. By convention, the later an element occurs when the table is traversed following the arrows, the more electropositive is the element.
[…]
IR-4.4.3. Formulae for specific classes of compounds
IR-4.4.3.1 Binary species
In accordance with established practice, the electronegativity criterion (Section IR-4.4.2.1) is most often used in binary species.2
[…]

For intermetallic compounds, earlier recommendations prescribed alphabetical ordering
rather than by electronegativity (see Section I-4.6.6 of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 1990, ed. G.J. Leigh, Blackwell Scientific Publications, Oxford, 1990).

Examples:
[…]

$\ce{Rb15Hg16}$
$\ce{Cu5Zn8}$ and $\ce{Cu5Cd8}$


Table VI as seen in [1, p. 260]:



Reference

IUPAC. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; RSC Publishing: Cambridge, UK, 2005. ISBN 978-0-85404-438-2.