It's actually pretty easy, but you may have to arrange a in series voltmeters setup, at the very least, in order to have a way to independently check out each link. For 2S, simplest way would be a DPST switch or a momentary pushbutton driving the coil of an inexpensive low power 5V DPST/DPDT relay. Example: cathode (B-) to first voltmeter's negative lead, midpoint (B1) to first voltmeter's positive and second voltmeter's negative (via relay), and anode (B2/B+) to the second voltmeter's positive terminal (via relay). Pressing the button would drive the relay coil from the battery, which engages the voltmeters in series at once over the battery terminals. Button disengaged -> voltmeters off, 0 battery drain. Quite simple.… I see no reason to take extra risk for a fraction of a second battery lag. The difference in performance just isn't worth the extra risk.
And/or get a suitable BMS. I recently ordered the 15A one here: surever9's 3A-15A 2S Li-ion Lithium 18650 Battery Cell BMS Protection Board 7.4V 8.4V Quite small.
The seller told me it uses Vishay Si4324 MOSFETs.
Cheers