SV,
I think your idea of cruising with another couple is a sound one. On the longer passages, you can split up the watches with each couple "on" for 12 hours as a couple. This also means that you can sleep together as a couple which is not possible with traditional watch schedules or less than four people on board. Since there are two people on watch at the same time, it should not usually be necessary to wake anyone for sail changes etc.
Another thing that a lot of people do not think about is that with two couples cruising together one couple can stay with the boat for
anchor watch/security while the other couple tours for a few days, and then you can switch. That way you don't have to leave the boat unattended.
For the above reasons, my wife and I have been talking for some time about taking another couple aboard our boat. We currently own an Irwin 38, and want to step up to something in the 48'-60' range to allow sufficient room for both couples.
As others have pointed out, finding the right couple--so that the dynamics of the relationship work--is all important.
Good luck with your plan,
Don W.