They are 70 psi pro pumps of eBay. Just out of interest why wouldn't you have two into one? Cheers
Even if you put a thousand pumps on there they wouldn't increase your water pressure.
If you put your pumps in parallel then they would increase your flow but 70 psi would be you maximum pressure just as Smithie says. If you put them in series you wouldn't increase flow or pressure.
Propump make a variety of pump sizes with differing flow rates from 2.6 lpm to 5.5 lpm.
Have you coupled your heater into the mains and tried it?