Why wouldn't 2 4gpm machines always work siamesed to together ? What can be some problems

How to properly siamese to 2 4gpm pressure washers. One is direct drive 13 hp 4@4000 and one is belt drive 15.8 hp 4@2200. I would have to turn the 4@4000 unloader to 2200. What should plumbing be ?

There are a lot of threads on here of people trying to do this. Haven’t heard of any one successful doing it. Just because you can, doesn’t mean you should

Sell both and get an 8/3000.

True, I used a weaker 8gpm and a stronger one.

What machine you recommending

I have a Pressure Pro with GX630 gear drive, the other guys seem to like the Udor pumps.