maybe its possible to use use a Voltagedivider
e.g. with 6KOhm and 1KOhm to GND.
So you would have +-1,42V measured at the 1kOhm to GND.
If you have a second Voltage Divider at the other (2.)power supply which gives you around 1,5Volts you could connect the gnd of the first power supply with the 1,5V so you should be able to measure around 0,08-2,84 on the first Voltage divider refering to the GND of the second power supply.
Maybe try this first only with two batteries and the voltage dividers, to see if it works.
I hope my explanations are helpful.
Maybe there is also an easier solution for this problem.
Do you have a datasheet of the resolver, or better a picture of how the signal looks and what you exactly need to know of the signal.
I think I did not understand what of the signal you need exactly or if its enough to know when the 0V crossing happens.