If you simply want to switch between the two amps then any passive switched like those recommended above will be fine. However, if you want to have the option of playing both amps together you three potential problems:
1) If you aren't using any buffered pedals in your signal path, by routing the signal to two amps you will reduce the impedance your guitar sees. This could load your guitar signal down and effect you tone by rolling of top end as well are reducing the signal slightly. This may not be a significant problem, but can be avoided by either buffering the signal our using an impedance matching transformer.
2) You may experience a earth loop between you amps which will cause hum. This may or may not be a problem depending on the power supply where you are playing. To avoid this you can either buffer the signal, use a splitter with an isolating transformer, or use a cable between the splitter and an amp with the earth discontected at one end.
3) You need to check your amps are in phase. If your amps are out of phase you will loose bass frequencies due to phase cancellation. If your amps are out of phase you just need to reverse the wires to the speaker/s of one of them. One point worth making is that on some channel swithcing amps the lead and clean channels are out of phase, which could certainly cuase problems in a multi amp set up!
Unfortunately sorting out problems 1 & 2 add expense to the project. If you do want to go down the "bullet proof" route Lehle make excellent, although expensive, transformer isolated signal swithing boxes. As an additional feature they are logic controlled, which means you can swith A, B or A+B with the press of one button, unlike a passive switcher.
Of course if you simply want to switch between the two amps a cheap passive switcher will be fine.
