as starting batteries only.
The Blue top is promoted as marine, and comes in two versions. One starting (or bow thruster/windlass) service only, the other for house use too

I have a friend who has used an Optima Blue top for 5 years on a Bow thruster with an 8 kW motor. That is 2-5 times the size of most diesel starter motors.

Thrusters do not run long, but they run longer than normal starter motors, so this says to me that the Optima would be good as a starter battery.

As I understand all the literature, it is nover good to try to charge two different types of battery in parallel, even if they ahve similar recommended charging voltages.