Those splitters sold by Audioquest are also sold at Radio Shack for 7 bucks each under the RS Gold line.
I use them to split out of my P-304. I currently have the output split between my M-504 and my Magnavox 8800. I have noticed no signal degradation what so ever when running the amps one at a time. I've never had both running at once, however, so your mileage may vary. In addition, you may have to have all of the components plugged in to the same outlet / power strip in order to ensure as common of a ground as possible.
As MacGeek said, you may experience a change in voltage and thus a change in SPL for a given volume level.
Some folks may argue that you may experience a slight change in sound properties due to the increased inductance and capacitance of the additional wire used in the multiple sets of interconnects. These figures are likely small enough to be negligible unless you are using exceptionally long runs of wire.