I have found that when I'm probing a circuit using more than one oscilloscope probe, clipping the ground of one of the probes to the board seems sufficient to give me stable readings from all of the probes. And this also reduces the chance of accidentally connecting two different voltage potentials through the ground clips of different probes.
I realize that the longer "ground loop" for the probes not directly connected to the circuit can affect some of the readings, but how? And at what acquisition speed does it become a concern? (I vaguely remember the original, analog scope that I was trained on in the military some 30 years ago only having one ground lead, separate from the probes. But I have no idea how "fast" that scope was.)