If 2 investigators are Lost in Time & Space at the same time, are they allowed to trade?

I don't think so. "Lost in Time & Space" is more of a status than a location. You're both lost but this doesn't mean you're together.

Tsuma is correct here. You're allowed to trade during your Movement phase; but while Lost in Time and Space you don't get a Movement phase. So, no

Thanks for clearing this up, my memory of AH rules is slowly fading.

Well it's not even just that they're not in the same physical place: they both lose their turns entirely.