Should the Unused Contact of a DPDT Relay be Connected in Parallel or Series?
One obvious disadvantage of connecting in series would be that you are doubling the contact resistance of the current path, resulting in more wasted power.

As you mentioned, by paralleling them you provide some redundancy which is useful. Additionally, ignoring the transient switching, you are essentially halving the contact resistance which should run more efficiently.

As @TonyM mentioned in the comments, you need to assess from a safety perspective which failure mode is preferred (fail close, fail often) but from a power perspective running them in parallel is preferred.