You can pay only once each time Underhanded Designs's triggered ability resolves. You can't pay more to cause your opponents to lose more life.

In a Two-Headed Giant game, Underhanded Designs's first ability causes the opposing team to lose a total of 2 life. You still gain only 1 life.

The number of artifacts you control is checked only as you activate Underhanded Designs's last ability. It's not checked again as it resolves.