1) Can you try and find the exact model of the original PSU please?
2) I wouldn't be surprised if it was power issues with a 'cheaper' PSU, especially now its working fine.
3) 2x S5s on an AX1200 should be fine, AXs are immense. There is always a risk that it may trigger its overload protection as found on some larger PSUs, which will prevent it from turning on at all though, its a bit hit and miss. Potentially another CX750M would be safer and more cost efficient for a second S5.
1)Seventeam ST-750PWL running both blades, then tried a Corsair PSU (CX500M) on one blade and either an Antec 500W or this one pictured below on the other blade. I'll have to look for the Antec box for the model number, it's quite old but brand new in a box. I got it as a replacement from Antec maybe 10 years ago, never used it.
The S5 is running fine now but it's only been about 5 or 6 hours on the new PSU, so if it lasts a week it will have broken its own record so far ;-)
Thanks for your help :-)
1) The first PSU is not suitable as it has 2 rails, which typically means you're relying on the PSU to properly load balance. In this case, its claimed each rail can take 30A on its own = 360W. That's really pushing it and likely explains your problems.
The second PSU is also not appropriate as its dual rail, again you're going to be relying on the PSU to load balance well to not exceed its ~200W on each rail.