I actually found Yrden to be more useful, there. Before I entered the ruins I imbibed Petri's Philter (to increase sign potency) and Tawny Owl (to regen Vigor faster).
I'd toss down a Yrden and wait for one wraith to get zapped by it, then go to work on the other one. If you're fast enough you don't even have to block and can finish off the 1st before the 2nd is upon you. If you can't finish the first quickly enough, roll out of their reach and rinse & repeat (Yrden, one gets zapped, go immediately attack the other and try to kill it quickly).
Yeah, the Yrden sign is awesome against the wraiths.