I believe phantom power will still work if you do it right. You could also consider plugging the two overheads into a simple mixer with phantom power and going out of that mixer into your interface. You could free up a few spots like that.

A Y-piece is unlikely to work, for two reasons. Firstly, the two mics will be fighting to be the sound source, and it will depend on the design of the mic as to what happens. At best it will result in a 6dB attenuation of the signal from each mic, and at worst it will cause clipping and distortion of the waveform from one or both mics.

The second reason is the problem of phantom power, assuming you are talking about condenser mics. The phantom power available from most pre-amp inputs is supplied through a 6K8 resistors in each of the + and - signal leads. This resistor value is chosen so that the voltage drop caused by the standing current to power the mic's circuitry does not reduce the available voltage at the mic below that needed for powering the electronics and polarizing the capsule. The designers of a mic have this resistor value to work with when deciding how to power their internal circuitry. As soon as you connect two mics to the single phantom power source, you double the amount of standing current, and all this careful design goes out of the window. You are likely left with under-powered mics that will overload easily or may not work at all.

As suggested, use a separate pre-mixer like a group on your main mixer if you want several mics to feed one channel (or a stereo pair of channels). Beware that small, cheap mixers generally do not make a very good job of highly-transient tracks such as those from a drum kit. Keep the gain trims low!