Longwave:
Use this.
http://www.mercenary.com/lilasitrdesm.html
I'm obsessed with guitar tone and this unit is AWESOME! It does exactly what you want, is inexpensive and you'll thank me once you use it. Really...
No loss in signal, nothing! I also use the Radial Injector and it allows me to combine/mix/match heads, cabs, blend them, etc.
FYI: Once I get a sound I'm happy with on each amp, I hook up to the Radial and blend away. Works great for re-amping as well.
If you're alone, try sending a DI signal (split with the Injector or Tone Bone) directly to the board/DAW and your amps. This way you can lay down some tracks, and dial in the sound to your ears, play with mics, etc. Then once you have it, document it and off you go.
Little Labs also has some great Re-amping stuff for a$200 as well (Red Eye) if you don't want to spend $750 for the radial or don't need/use like 6 or 7 amps ( Injector can handle 7 Rigs).
Keep in mind phasing issues (Little Labs IBP or Jr) and you're rocking!
BTW: The IBP can re-amp as well as keep things in phase!
Good luck!
TJ