What they mean is if your mono amp has 2 + and 2- outputs you can parallel the coils on the subs then parallel one sub to a set of + and - terminals. And do both subs in this manner.
The reason this is... is because the two sets of output terminals are actually wired together in parallel, so youd effectively have the same wiring.
This would eliminate running a wire through chambers in your box, as wlel as cramming three wires in one post on the subs. the two pairs of terminals are there exactly for this reason, to make the wiring more flexible and clean.