Can two primary drain lines be interconnected?
What about two secondary drain lines?
In a word, yes.
If combined they should be resized to compensate for the increase in water flow and possible slugde build up.
Keep in mind if it plugs up, you have two units that will over flow.
But, it is always better to drain each one by itself.
That is the best and most reliable method.
If you try to fail, and succeed.
Which have you done ?