You have to make a cost arbitrage.
How much more expensive is the 1200 W PSU (compared to two 750 W)?
Because running 3 cards instead of 6 will cost you $10 a day. Suppose the 750 W is back in stock in three weeks, that's $200.
Didn't think of that actually, I will be able to get the second 750w in a few days I found it in another city and I happen to be passing it on Tuesday, not sure if my build will be finished by then anyway so think I will go with 2x 750w. The 1200w is out of stock and not a sign of it returning anytime soon.