is a 1300 watt PSU enough to run two S5's?
Yes it is, as long as its not a terrible brand.
is a 1300 watt PSU enough to run two S5's?
It should, but it's close to boarding on it not (only 10% headroom). I would think it's OK, but would keep on eye on the setup for a while before feeling comfortable leaving it unattended.
S5s are 590W at the wall with a ~85% PSU. That means 500 pre PSU. 1000/1300 = plenty of headroom.