Thanks to everyone who has answered my question
here.

My question was a bit vague. I was in the context of “
two channels with invert-and-add” and I should have been more specific. My point was not to float the scope.
Now I've played with that a bit.
First of all, on a DSO, Math: subtraction is probably easier than invert+add. That amounts to the same thing, doesn't it?
Then I played with my FY3200. It doesn't come with a 3-wire cable. I made one myself, whereby I can switch the PE on/off.
If it is grounded, then the 2nd channel is of course without signal. So this method is useless.

If it is not grounded, then there is so much voltage on it (90Vrms) that you can hardly see the signal.

However, it works really well with a battery-powered signal generator.
