I have two pumps going through the heater just make sure you fit in this order
Tank 
Pumps
One way valve of each pump into t piece 
Then through heater
Then out heater into t piece 
Then to both deals with a tap before one real to even flow on both deals 
One way valves are so the pumps don't fight against each over and pump back on eachover which they do lol one way valve is esential 
If you require pics please from me an email 
deanlawrence89@hotmail.co.uk