If you use a splitter, then there would be a slight drop in signal to each sub, but you'd need to re-calibrate your subwoofer settings in the receiver anyway upon adding the second sub - the drop will not be something you notice.

There are arguments either way on a single sub vs two or more smaller subs. If you already have one sub and want to add another identical, doing so may be easier than selling the existing sub and buying a larger sub. Two subs can also help counteract room effects.

If the pre-amp has two sub connections, I would use them both rather than adding a splitter to one of them. The signal path would be identical (same information going to each sub), but I suspect the two connections at the pre-amp would not suffer from the signal drop of a splitter.