No clue. Was told it was brownish-black when found.
Hi, welcome to Spider ID. 🙂 This is a Longlegged Sac Spider (Cheiracanthium mildei).
If I put a yellow sac spider in with a Longlegged sac spiders habitat, what would happen?
Generally speaking that is two different common names for the same species. Though the names may also be used to describe other species in the family/genus at times. What happens would probably depend on the maturity and gender of the spiders. They might mate or if the enclosure wasn’t big enough for the both of them, one might eat the other.
How big should the enclosure be if I didn’t want them to kill each other? And how do I tell if they’re a boy or girl?
I would just go with separate enclosures if you don’t want to risk them eating one another. If they’re immature you can’t determine gender. A mature female will have a visible epigynum on the ventral side and slender pedipalps, a penultimate or mature male will respectively have swollen or sclerotized pedipalps.
Immature male: https://bugguide.net/node/view/898690/bgimage
Mature male: https://bugguide.net/node/view/906999/bgimage