Can you use a word ending in -phobia
as a plural, in the following context:
Their anachrophobias led them to running away from the fake spider.
If it's a shared characteristic, there's only one of them... so, their phobias... would be one thing, but their (shared) arachnophobia... is something else. There's only one phobia, after all.
I think there's only a reason to use a plural if there's a material difference that makes the (thing) different. So, their skin colour meant they burned easily versus their skin colours... is simply a way of saying if they were identical or not. Given that you're talking about a class of phobia that covers both people, the singular would be appropriate.
(c/f their nationality if you want another example - if they were both German, you wouldn't say nationalities, as there remains only one nationality in play).