If you intend to use multiply pumps in a single system it is worthwhile considering what configuration they should be connected in to produce the maximum flow around your system. You have two choices of configuration.

1. Series
2. Parallel

Series

When the pumps are connected in series, they are connected one after the other. This should roughly double the pressure while keeping the flow rate about the same as one pump.

Parallel

When the pumps are connected in parallel, they are connected next to each other. This should roughly double the flow rate while keeping the pressure about the same as one pump.

When adding an extra pump to your system it's important to consider that you are also dumping twice the heat into the loop than if you had only one pump. In some cases the reduced C/W due to increased flow rates doesn't overcome the increased heat added to the loop due to the extra pump.

Which configuration is best once again depends on the back pressure of your system and the flow pressure relationship of your pump.