Beware of youtube videos about connecting 2 compressors together

Bill your photo is really too small, one problem I see is "no strain relief" with the rigid pipe from the upper to the lower compressor pump

Also the plumbing routing may be wrong. You may HAVE to use one more check valve if there isn't one----from the upper pump.

Personally I see nothing wrong with the idea, 'run what'cha brung.' I have 3 compressors, all bought used. One works well, but I use it little, it's a high speed direct drive. I use it for "more when I need it." The other is also direct, and the tank leads at one of the plug threads. "I was always gonna fix it" so it sits there. The "main" one is a belt drive I picked up, all that was wrong..........needed a belt and new switch. I think I paid 80 for it 2 1/2 hp