Hi Dasherbun!
This also happened to us when we got a second rabbit. Like Sirius & Luna said – from what I have seen this is very normal! Our first rabbit (Jay) is a very assertive little guy – he also thinks he is the King of our apartment. Every time we would let our second bunny (Lola) out, Jay would get so aggressive; he would pick up his pen and throw it down, start biting at his rug, all sorts of stuff. We were worried he was going to hurt himself.
This was about 6 months ago; since then, he has calmed down a lot since Jay has accepted Lola as part of his life.
I know that’s not a great answer, but over time your bunny will settle down. Your first bunny is probably just very anxious that there is a second bunny in his territory.
We let our first bunny out a lot more than we did our second bunny at the beginning – this helped with his aggression. Although it’s not technically “fair”, it was what we had to do to keep Jay from being nasty.
The bunnies should be let out one at a time at this stage – it’s a really scary thought to have two bunnies out at once if they aren’t bonded – they may end up getting hurt!
I also suggest providing your fist bunny with a lot of toys as a distraction when the other bunny is out.
Hope this helps!