Is my snake actually a female?

I had my supposedly male ball (~1000g) out on the table with my other smaller known male ball (~300g) for a photoshoot. The smaller one started moving around the bigger one and I freaked out because I saw something red at the smaller one's vent and thought it was a prolapse or injury so I quickly picked him up. Turns out it was actually the hemipenes because he immediately retracted them again when I picked him up.

Was he actually trying to mate with the other one? Does that mean my other snake is actually a female? I obviously don't want to breed them at their current size.

Seems like it, get it sexed to be sure

Not necessarily.
They will sometimes do it around other males. You would have to probe or pop the other one to be sure.
This is funny though because it's actually exactly how I discovered my first ball was a female. She was actually presenting to my small male while I was trying to take pictures of them and I totally thought she was trying to poop on him. I freaked out and grabbed him away from her and she kept moving and doing a little tail wagging dance. So I looked it up and was very relieved. Lol

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Whether it's trying to mate with the other or not does not matter and will not tell you if the other is a female (males will lock with each others)

You need to get the animal sexed