Pair programming adaptation

First explained by Kent Bent in xP, Pair Programming (a technique when two developers work together on same computer) mostly become popular because of developers.
Yes, it's fun to work in pair. As for effectiveness of this method there are a lot of research of this area.
By economy effect I mean ability to produce a certain number of same quality code by less time.
There are a few factors that should be involved:

• Max time developer can active work can't be 8 hours/day and 40hours/week
• Number of bugs/suggestions can be instantly found during coding by second part

That's seems to be important improvements, but our target is at least x 2 effectiveness improvement as we reduce number of developers.

My experience says that we also should take in consideration project itself.

• Pairs work great on projects with tough deadline limitations
• Project technology and ways to implement should be clear in general - not for research projects Its
• Pairs should be under constant control by project manager to avoid stuff chats, pairs should be compatible in sense of psychology.

The third limitation seems to be the most important