I solved it with a 2x 100Mhz battery oscilloscope, which is enough for my purposes
This sounds like an ordinary oscilloscope with GND connected to all probes?
It still has most of the problems:
- common mode of both channels must be the same
- loads the "negative" input with capacitance to ambient (even more so when you are touching the device)
- can be dangerous if common mode is high voltage with respect to earth
It eliminates one disadvantage of "ungrounding" a mains scope:
- no mains noise injection through capacitance of the PSU