Doesn't sound right to me. My Victron Phoenix 350w inverter charges my Makita batteries without problem, one at a time using a Makita fast charger. Initially I had a 30 amp circuit breaker but it would keep tripping it, & checking showed that it would pull up to 32 amps briefly. I fitted a larger circuit breaker & have never had any problem charging the Makita batteries since. By rights 32 amps is pushing the limits of a 350w inverter in a 12v system, (at 12v it equals 384w, at 13v it equals 416w) but Victron under rate their inverters & I have tested mine at 500w for 1 hour & it just kept going. It probably weighs much the same as 3000w one because it contains a toroidal coil - which according to folk who know more than me is a sign of quality in an inverter.
If an 800w inverter couldn't charge a single 18v Makita battery on fast charge there must either have been something wrong with it or the label was telling porkies about it's capacity, but if drawing double the amps (64 amps) on an 800w rated inverter with a dual charger , especially if the inverter rating was marginal at best (or over-rated as many are) then it would be understandable that it couldn't manage. 64 amps at 12v =768w & at 13 v =832w.
It would be interesting to compare the cost of my inverter with your 800w one & your 3000w one, although of course if you need to charge two batteries at once my one would not suffice.
Current cost of my one is $170 with a 5 year warranty.
https://www.ebay.com.au/itm/37305936...frcectupt=true (actually 375w - not sure if they do the 350w any more)
The 800w version (also with 5 year warranty) & I'm confident it would charge the two batteries at once, is around $470.
https://www.ebay.com.au/itm/27447389...frcectupt=true
Pic of my inverter & my Makita charger.
Attachment 84473Attachment 84474