using double powersupplies
Why do you "think that one PSW could feed into the other"? ORing diodes is pretty much a standard solution to prevent this, recommended by PSU makers, e.g. http://www.kepcopower.com/support/diodes.htm. As long as the diodes are properly selected so they don't overheat, I don't see an issue here. You could use Schottky diodes to reduce the voltage drop seen by the router and simultaneously decrease power dissipation on themselves, but since you said it already works with the diodes you've tried... it seem a bit pointless to bother now.

The mismatched voltages are a problem when you want to load-balance the sources on a load (e.g. see http://www.codemsys.com/smps/Parallel.htm), but that issue seems irrelevant to your application as either PC source can presumably power its PC and the router (presumably a home model) comfortably.

There are slightly more expensive stock IC solutions for redundant power supplies, which is basically what you're doing from the router's perspective, (e.g. see http://www.ti.com/lit/an/slva094/slva094.pdf), which have some additional advantages like almost zero voltage drop to the load, but they seem overkill for your application.