If you don't mind losing your second channel, you
can always apply one input
to channel A and the ground reference to channel B, then set the scope to
"A-B" mode (ChA subtract ChB). Most scopes can do this, and this is exactly
what it's meant for.
If you do this, make sure the 'scope can stand a 'common mode voltage' of
a few hundred volts, because that's what it's going to be seeing. I would
think a fair number of 'scopes will not be happy looking at a signal of a
few volts (so the Y attenuators are set to, perhaps, 1V/cm) with a common
mode voltage of the full mains.
-tony