I actually have just been to a meeting about this, as it related to HSV 2. Superinfection , that is, being infected with more than one strain of the same type, appears to be unusual in the US and in people who are HIV negative. So in summary, I would not be concerned about you contracting a new strain of HSV 1.
In terms of your partner, I am not as concerned about her risk of acquiring HSV 1 with the addition of this new partner as you are not in a different position, really, than you have been. The screening test misses 27% of HSV 1 infections, comapred to western blot, so her testing may or may not be accurate anyway.