Try to use the lower foil only. If it works, you have a spare one. You do not need to connect them both.
Of course if the solder of the E-side foil connector will break, you need to exchange the foils for a better (acceptable) sound, but during the gig you can play the set to the end with the G-side foil only and care for the foil exchange during the break.
Just try out soon how the E- and the G-side foil sounds when only one of them is connected to the battery adapter. The change in sound will give you a hint before both connections might get broken. And maybe check every month or week (depending how much you play) both foils (by unplugging the other) if they still work.
If you soldered the connectors well, I don't think they will give you any problems in the future. I'm trying a lot of strings during the last time, so I needed to disconnect and reconnect the plugs to the adapter rather often and nothing problematic happened yet.
I'm not not too much in trouble if the Nanoflex wouldn't work, since I have a Full Circle and a Shadow bridge wing pickup on the bass too. But even if I haven't the only thing I would need to care for is a spare battery, as you already did.