There are 2-speed motors that run on 120V ... a little harder to find. Running a motor on 240V will not save you any power costs, just will lower the amp draw over the wire. Although you need to make sure you are not trying to pull too many amps over too long of a distance. But, if you are currently running 15A .... the 2-speed will not be any higher.
You can either have a manual switch for the speed, then when you would typically want solar, leave the pump on high speed and the timer on during those times. And when you know you do not need solar, put the pump on low speed with the solar off ... that will be when you save the most money as running the solar on low speed may not be possible ... and the heating is more efficient at higher flow rates anyways.
Or you can add a timer to select the speeds.
Some of the solar controller automatically will switch to high speed when solar is needed (not sure about the one you have though). So you would set your pump to run on low during normal solar hours and if the solar was called for, it would automatically switch to high speed.