The idea of them fusing completely in both body and mind to become a single creature sounds interesting, also the part about becoming a chimera beast (even if it doesn't say anything about which animal. I must admit that the most interesting animals for me are the hoofed ones).
The penis idea is not new, but rare enough to make all such stories enjoyable. I also like the circular, spider-like being with their heads reduced to eyes.
My suggestion for a fusion; they continue to fuse, like a magnet has been placed between their shoulder blades, and the back of each hand is touching each other and fuse before being absorbed by the body. At first they are simply a single torso with four legs and their heads facing in each direction. Then their breasts starts to be absorbed and disappear into the body (alternatively; only one breast disappear, the other get divided into two separate ones). One of the heads start to sink down and be absorbed, until only the face is left on the chest, and keeps travelling further down till it ends up between her legs, possibly only as a pair of eyes.
The other head starts move forward and to sink too, and ends up as an upside down face on the top of the chest with no neck. Her legs starts to migrate further up the back, and ends up about where the arms used to be (but they don't walk on all four, and are still bipedal). Maybe the genitals and anus disappear only to the leave a smooth crotch behind, or they could fuse with her face in a similar way as with her sister.
It's hard to decide, but I think I'll go for the circular spider thing.