I don't have much personal experience with it, but have read certain BMSs don't like being in parallel. Assuming that's not a problem, then diode free is better, otherwise the packs won't try to equalize voltage with each other. If they aren't perfectly matched units, one will have slightly less capacity and end up at a lower voltage during use, and the charger will get confused trying to charge the two different voltage packs, and I think it would be counting on one of the BMSs to cut the charge to one pack while the other one is still filling?
- I end up with a lot of functional packs with dead BMS, so do a lot of diode-less paralleled up bulk charging of slightly different recycled packs, with occasional manual balancing, so self equalizing during use between packs is important for me, but that's not the safest solution for most people.