This is a stretch idea, probably not workable, but I will jump off a cliff ....
All CMOS inputs have a threshold. One could implement a PWM, then filter it, to
get a DC value. Then apply that to an input of interest, use code, and determine
the PWM value, hence DC value, the input trips at. Additionally one could use
a simple inverter stage, strap internal across it a high valued R, which biases
it up into a linear amplifier stage, "linear" used loosely as an adjective. And use that
as a G stage if needed. So at this point you have an input that trips at a value determined
by PWM, eg a sloppy comparator. Implement two of these stages. That allows phase
determination.
Obviously there are a number of T and V and noise issues and I am sure other issues to deal with.
Just a thought.......
Note you have not mentioned freq content of input signals.
Regards, Dana.