I have a 10,000 gal pool (big Spa) with a Pentair WhisperFlo 2 HP (1.3 SF) that runs 90 min daily.  My flow meter shows 110 GPM through the filter/heater.

I also have a second identical pump used as a booster that goes around my filter/heater to a Y coupling with 2 check valves which goes to my pool jets.

When I turn on the booster pump the flow measured through the filter/heater drops from 110 to 65 GPM due to the back pressure from the Y connection.  This is OK because the heater's minimum is 40 GPM.  The blast from jets is great with both pumps running.

My only problem is that my filter pump motor bearings are getting noisy (12 years old) and here in California I can't repair it.  I MUST replace it with a variable speed unit.

Reading on this site, I like the 1.65 THP variable speed pumps which offer external control wires and remote control with V-Link. Interestingly, the bigger 2.7 THP pumps don't seem to offer the external control wires, which I need.

I'm worried that due to the Y coupling, the weaker 1.65 HP pump (even at full speed) will drop below the heater minimum of 40 GPH because of back pressure from the more powerful 2 HP booster pump.

How can I know in advance if a 1.65 HP pump will be able to maintain at least 40 GPH through the heater when the 2 HP booster is running?

Also - will I get any benefits from switching to a variable speed pump since I only run the filter 90 minutes daily?