Go ahead pal - use 2 phantom sources...
The whole idea is so fucked - I dont even want to try...
Do you know anything about electric circuits when you say this?
By using a splitter to split the electrical signal to the mics, you create a parallel circuit. In a parallel circuit, the voltage on each side of the circuit remains the same and the current varies. Thus, applying +48v phantom power to both sides of the parallel circuit is not problematic in the least bit. The available current increases (since both the V3 and the PS2 are supplying 48v phantom), but the mics will draw what current is necessary, so the additional available current isn't a problem at all. (In fact, it could be a benefit if you needed to power current hungry mics like Earthworks.) Both the V3 and the AD20 are expecting +48v phantom at their inputs, so they use DC blocking capacitors to block the DC phantom voltage and allow the AC mic signal to pass. This is exactly how phantom power is designed to work. So to reiterate, it isn't a problem at all to have phantom on from two sources. No different at all really from wiring up two batteries in parallel to increase your runtime from using one battery.
It'll depend somewhat on the circuit design, but changing your levels on your preamp will change the impedence of the circuit. This slight change in impedence isn't a problem either, but it will slightly change the mic signal levels the other preamp is seeing, thus the impact of changing levels on one unit on the other. Not a problem, just inherent to using the mic splitter cables. You could solve this by using a transformer based mic splitter. In fact, if you used a transformer based mic splitter, you would absolutely need to run phantom power on both preamps since a transformer will not pass DC either. (The m148 for example does not use blocking capacitors, but instead uses transformers to block the 48v phantom power.)
Bottom line, this idea isn't fucked at all, it is perfectly reasonable. The only thing that is fucked is your advice.
EDIT: Looks like Matt got this info out while I was typing up my response. Yep Matt, you've got it exactly. And yes, I do have an electrical engineering background.