Are you dead set against using two meters? I have
one with RS-232 capability (Radio Shack). Not too
expensive. You would have to poll them separately.
What is your input voltage range? Many available PC
data acquisition systems out there, including USB, for
not-too-expensive, some offer common-strobe sampling
if that matters to you (it might). You might be able to
use voltage dividers and get more usable range at some
cost to resolution / accuracy and so on.