The check valve is there to prevent backflow from the discharge pipe (which is usually vertical as it leaves the sump) from filling or partially filling the sump well back up.
There are two main failure modes for a check valve - stuck closed and stuck open.
If the check valve is stuck closed, then the sump pump will not discharge the water when it runs. Worst case the water will just circulate continuously and the sump pump will run continuously.
If the check valves fails stuck open, then the pump will discharge the water in the sump, but what remains in the vertical portion of the discharge pipe or the downward sloping (back to the sump) part of the discharge pipe will flow back into the sump pit.
So with two check valves in parallel, you protect against the stuck-closed scenario (as it is unlikely that both check valves will fail in the same manner), but are actually more likely to have a stuck-open fault, since if either check valve fails open the discharge water can flow back into the sump well.
OP changed his question to mean two check valves in series, not parallel. So what Jack said.