If I connect two iPhone batteries to one bms in parallel would it work

Jack Mitchell For Educational Purpose only, right? Anything you build based on this is at your own risk. So if you torch the house down or burn your face don't you be saying it's because of this :-)

In theory that should work. You need to remove the BMS from the batteries that you are using. Then connect both to one BMS. The big problem you are running into is that the BMS with those 2 batteries does not guarantee to share charging and load currents evenly. You may have some major charging disparity and your BMS may have difficulty balancing those.

If you connected them with their individual BMS intact you'd run risk of if one battery is at a different SoC than the other pack, then the stronger battery will try to charge the weaker one. This could lead to over current in the parallel link and cause major issues with heat etc. Could definitely harm your batteries. The other problem would be that one battery tries to shut down because of a low charge and the still charged battery could re-open the BMS of the one that tries to shut down due to low SoC, and thus compromise the protection for the low battery.

Maybe a diode (or even better an FET because of its low resistance) in between each pack to ensure the reverse current would be stopped....(point to ponder)

Wonder what happens if you connect both complete batteries to a combined BMS.....it's the balancing of the SoC for each battery that throws this off.

Don't blow up the house or put your phone on fire. I hate venting with flame.