I have never encountered any double boss before in all my years playing PD*! And that's actually saying something, because I play Shattered quite often. So, in my inexpert opinion, pretty darn small!
* limited to Vanilla, Shattered, and Experienced. Also bits of Yet Another, for a little while about a year ago.
Oh dang, this is the 4th floor? I didn't even see that previously!
In that case, I've encountered double caustic slimes quite often, and wouldn't really be surprised to encounter them at all.