Caveat: I own a AC200 MAX I do not own a B300
I’m assuming (a dangerous thing ) that when you say you have 2X500W chargers, you’re referring to the T500 chargers. In that case, I’d play it safe and suggest one of two courses:
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Charge the AC200MAX+1 B300 (connected) to a T500 and charge them via the AC200MAX to full capacity
Charge the second B300 separately using the second T500 to full capacity
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If you have solar panels or another charging device compatible with the AC200MAX and the B300, the Bluetti products support concurrent charging from solar and wall AC. A single B300 can take up to 700W total charging - from the grid (T500) + 200W solar. The AC200MAX MPPT controller will accept up to 900W solar and up to 500W from the T500.
You can even charge each B300 separately with T500 plugged into each one, while charging the AC200MAX via solar panels. Very flexible charging capability. I love my AC200MAX.