This comes up often enough in our work that it is worth a post.
When two or more heat pumps are pumped in parallel without thought given to flow, you can run in to some issues. Look at this figure:
If only one heat pump comes on the flow can go backwards through the second heat pump and return directly to the one heat pump. This can be on either the source or load side of plumbing (if radiant). Worse yet, this may only occur intermittently. I’ve seen systems in place for years that have been plumbed this way. It causes problems “sometimes” and may or may not be noticed by the owners. But it does cause some extra wear and tear on the units due to the extreme water temperatures that they are now exposed to.
The simplest way to solve the problem is to use check valves on the supply side of the heat pumps.
There are other options with pump locations, solenoid valves, and plumbing layout.
I have just shown enough to get the concept across as it does put us in a tough position if we end up servicing such a system. There is not much reason to fix something wrong with the heat pumps until one fixes the plumbing.