Can't do it
I don't think you can put pumps together in series like that.
If the first pump isn't putting out enough pressure and volume, the second pump will starve on the suction side and won't do any good anyway.
If you have a dock water hose connection hook that up and try your shower. If the pressure is low I'd suspect a constriction somewhere. If the pressure is fine then I'd suspect your pump.
Pumps in series don't work.
Your problem is likely owing to a little known fact.
Stern shower hoses have a check valve built in. This is mandated, as they are used in RVs. If the shower head accidentally fell into a bucket of poisonous crap, you wouldn't want that siphoning back to your fresh water tank.
So, a check valve is put in.
If you remove this check valve, your pressure will increase.
Also, make sure no kinks in the hose