Any tips for splitting bath bombs?

In the past I have just banged the bomb against the tile floor. I have to give it a really good whack to get a clean split. I've tried cutting them as well, but that just made a huge mess.

I find placing the tip of a knife exactly on the seam and then giving it a gentle whack with a hammer always works well. This even works on bath bombs that have no obvious seam.

I just bang the bomb on the side of the tub, along the seam of the bomb. It usually splits right in half for me.

I usually just stick a butter knife in the seam, it works pretty well for neatly splitting them in half. I leave the bomb in the bag to catch any crumbs just in case it gets messy :)

I often split my bathbombs. Partly becuase I have a small tub like you and partly becuase I am cheap and want to get more out of my lush :-) I use the back of a heavy kitchen knife. I leave the bath bomb on it's bag (so I can catch any crumbs) and place the bomb on a folded kitch towel. This give it more stability. Then I give the "seam" of the bomb a good solid whack with the back of a kitchen knife. Sometime they split easily, other times, it might need another wack. I think the freshness of the bomb has to do with how easily they split. The more fresh, the more soft they are, and require a bit more persistance. Good luck!