Why would you want to run them in series?
I suspect you mean to connect the outputs in parallel so you will have something like 215A available (according to your theory). Since the units are probably normal polarity, constant voltage DC, and share a common ground, I suspect you will get into trouble as the two units are not matched and maybe even if they were. That is, what would happen if the 125A unit were outputting 30V and the 90A unit were outputting 25V?
From a practical standpoint, the gun for the 90 is probably not built to handle wire for 215A. So, at best, you will end up spray arcing.
Never tried to do what you suggest. I don't suspect there are many who have. So, I would be extremely cautious to the point of not even trying it, unless someone can report hard and fast results from doing what you propose. You could end up ruining one or both welders.
John