Two houses together,each with their own pump. Spliced into one line with a Y connection and from there one line out to the ditch. I would also install a check valve before the Y connection so water doesn't pump from one house to the other. Would this work?

What are you pumping out to that ditch?

I would also install a check valve before the Y connection so water doesn't pump from one house to the other. Good idea, wrong set up! To prevent one pump from pumping back to the other house you will have to install two check valves.
One before each branch of the wye,( lay it out on paper). I would also increase the size of the pipe from the wye to the ditch up one size.''.
Good luck, Tom

Are you pumping towards the house or away from the house ? Water or Sewage ?

Miloi,
The way I'm reading this is they are discharging sump pumps from two houses out to a common drain line. I can't imagine pumping sewerage from two houses out to a roadside ditch. And your take? Regards, Tom