If you’re measuring only voltage, you can use A-connector as common and your double-bananas should fit. After all, there is only really small resistance (and fuse) between C and A.

Or actually.. I have measured 70/100V audio with both channels, in hope to find impedance.. Amplifier negative to C, speaker negative to A and both positives to V. Mooshimeter might be able to tell us something at 1kHz, like most impedance meters.. Maybe something for the app-side wishlist, even after @admin has told me its impossible ;)