It’s not that two of anything is treated as singular. It isn’t. If you have two of some things, you use plural in Gaelic.
It’s that the numeral dà takes the singular form (actually historically dual form, that later became the same as singular dative, and that recently generally went out of use). But it’s just that, you use the singular form directly after the numeral ‘dà’, nothing more to it. You still refer to two things as iad, and not e or i, you still use the plural when not using the numeral, etc.