Not sure you'll need 2 sockets. We're on the road at the moment, and have one socket on one bike only - more than enough, although we're not using GPS.
We've found we've normally been able to find somewhere to charge our kit along the way, but those times when we have days away from civilisation I've got a really simple 12v cigarette style socket which i bought from Hein Gericke for about £10. I bought a small really lightweight mini inverter from Maplin (i think for about £15 or so). I haven't bothered drilling the socket into a fixed position on the bike. It lives under the seat, connected to the battery, and when i need it I run it out and into the tankbag or other bag, connect it to the mini invertor (which lives in a small side pocket of my tank bag) and use it to charge cameras, laptops,ipods and anything else.
As for your GPS, you could just wire that into your bike and forget about it. Certainly, with what we're charging we've never had the slightest issue with the battery and nor do i expect to have one.