The 48VDC phantom power is not going across the hot and cold signal connections of the mics preamp it is between pins 1 (-ve) and pins 2 & 3 (+ve) equally so 2&3 are at equal potential (meaning there is NO voltage across them, this is why it doesn't immediately destroy any dynamic mics connected to a live P48 input, don't connect your vintage ribbon mic there though). If you used a TSF jack to XLRM adapter to connect an old, jack terminated, dynamic mic to a P48 input you would almost certainly destroy the mic. With capacitors any signal sent by mic one between pins 2 & 3 would be driving the output devices of mic 2's preamp, as the output is very low impedance compared to the desk's mic inputs that would generate a much greater current into it. Most will handle this fine but it is definitely not what they were designed for and I wouldn't take the chance of (at best) subtle distortion and at worst damage. With dynamic mics the issue of damage is minimal, they are pretty robust devices but, if you were to connect one to another then excite the diaphragm of the first, in theory, you would hear the second behaving like a loudspeaker (remember the "two tin cans and a piece of string walky-talky beloved of pre-teens wannabe 007s"? this is just the electrical version). This is exactly what you are doing when you use a y-cable to send two mics to one mic preamp. This can't be expected to sound good.
