I would guess that it would be cheaper to replace the motor (only the motor, not the entire pump assembly) with a 2 speed pump than it would be to buy and install an entirely separate pump. I know that you can buy the 2 speed motor for the Hayward series of pumps online at some pretty reasonable prices.

Normally, though, solar is run at high speed, not low speed. There is another discussion going on about running solar at low speed. You can run solar at low speed, but in order to do that, you normally have to prime the system at high speed to get water up to the panels and completely purge the system of air. Then you could change to running at low speed. On my solar controller, there is a relay that can be used to kick a 2 speed pump to high (or to run a separate booster pump), I used it to kick my system up to high when solar is on, then it goes back to low when solar is not running.

I think that with a more expensive controller, it would be an easy thing to program to run at high for a bit, then kick down to low, but I'm not sure. My Ikeric pump has the ability to start in high speed for a while, then kick itself back down to low (this is normally used for pressure side cleaning systems). I haven't played with that since I don't have that requirement.