Check the output port on your sump pumps..most of them have a 2" port with a reducer that will permit connecting an 1-1/2" line. For peak efficiency they should be connected with the largest pipe size that the pump is set up for.

The standard rule of thumb is if you increase the size of a pipe by one nominal trade size you double the capacity..so if your pumps do in fact use 1-1/2" you could combine them on a 2" line.

If they will accept a 2" line then combine them on a 3" line.

Make sure you have a 1/4" per foot pitch from the highest point of the line above the pump to the point were it exits the structure.