I would think that keeping them separated would be the best option. If one goes weak, the stronger pump will back pressure the weaker one to a degree. I don't think you can do anything with valving because the system is pressurized.
Generally, in multiple pump setup's (multiple sources, same outlet), one is for backup, or you cycle them.
You may be able to hook them up as source and booster, then feed the manifold.
We'll see what someone else has to say about this.
