We'd have to know more about your plumbing to give the best answer. And perhaps there is an unobvious reason that your solar heater system was plumbed that way, but normally (at least here in the US), it's just not done like that, at all.
Firstly, and I'm assuming your solar heating panels are typical, you would never run unfiltered water through the panels. If they're typical, they consist of hundreds of tiny little tubes that are going to get clogged up by who-knows-what-crud pumping through them.
Secondly, having the heated water return through only one return is going to inhibit the heating of the water, the very thing you're experiencing.
And then there is the matter of running two pumps when the job of filtering and heating is typically only done with one.
Sometimes a second pump is used when there is a good enough reason. Perhaps the run to the panels is long enough or high enough to warrant it. Can you describe the type or brand or model of solar panels you have, and where they are, physically, relative to the pool?
All that said, the typical and most efficient arrangement is to intake water from a skimmer or two, using one pump that sends the water through a filter, then on to the solar panels, then back, through the SWG, then into a single manifold that then distributes the filtered/heated water to all of the returns. The very best setup would include a dedicated valve for each return so that one could adjust the flow of each. And finally the water would enter the pool through adjustable "eyeball" returns that allow control over the directlon of flow.
With that set up, you could adjust the flow rate and direction of each return, and direct the heated water evenly around the pool.
IMO, that's how you should replumb your entire system, and then retire one of the pumps (save it as a backup).
Unless you can describe why it was necessary to use that second pump in a separate "heater loop."
