I've been on the search for a while for the best DJI Osmo battery charger that wasn't an OEM product. I found several that are included with different battery packages that claim to charge two batteries at once. Of all the ones that I found that can charge two batteries at once, only one of them so far is able to charge as fast as the original charger but with two batteries at a time. That means I can charge one or two batteries at the same time from 0 to 100% in one hour.
Many of the others only charge at 40% to 50% the rate of the original charger. Granted if you were charging two batteries at the same time at 50% rate, you would effectively have charged both batteries in the same time in that slower dual charger as you would have if you just had the one charger, except for the convenience of plugging the batteries in and coming back two hours later.