Sometimes just one amp isn't enough. Using multiple amps together can yield a sound that is more than the sum of it's parts. Both Corey and myself have split our singal to multiple amps. Currently Corey runs both a Fender Twin Reverb and a Savage Blitz 50 simultaneously.
We've tried many numbers of splitters searching for the best way to do this. When I was using a Sunn Model T in addition to my Marshall I was able to use one of the front panel outputs to drive my Marshall. This added a bit of extra gain to the Marshall so it wasn't ideal.
Another common way to split is an ABY switch. There are dozens of them out there and they are a great solution. Especially switching back and forth between amps. The issue with the ABY is you could pick up ground loop or phasing issues. Several ABY switches had ground loop lifts but very few had anything for phasing.
The solution Corey and I decided on is a dedicated splitter designed for guitar. We aren't running racks so we didn't want something rack mounted. We didn't need the flexibility of switching back and forth; the amps are kept on at all times. We found the Lehle P Split. The P Split handles ground loop issues and phasing. It's passive and requires no power. It's the perfect box for our uses.
It's also great in the studio to cleanly split your signal to two amps. Can never have too many guitar tone options!