I was curious about this, so I took a look at the game logs. Dominguez has been asked to come back out after pitching a full scoreless inning 8 times (I ignored a number of appearances where he got the last out of an inning, then came back out for the following inning):
5/19 - scoreless 2 inning save
5/31 - scoreless 2 inning save
6/6 - scoreless 8th, then BB, K, single in the 9th - Morgan gives up a grand slam to Jason Heyward 3 batters later
6/10 - scoreless 7th, gives up a run on 3 singles in the 8th, but escapes and Phils win 4-3
6/27 - scoreless 2 inning save
7/9 - scoreless 8th and 9th in a tie game - Arano gives up a homer to lose in the 10th
9/5 - scoreless 2 inning appearance
9/11 - comes in with 1 on, 0 out and escapes, then meltdown in the 9th
So in 8 such opportunities, Dominguez has a clean 2nd inning 5 times, 1 successful tightrope walk, and 2 meltdowns. If you look at all his other appearances where he pitched in more than 1 inning (but didn't meet the earlier criteria), he has 8 scoreless 4 out appearances, then 8/25, where he got the last out of the 7th, then fell apart in the 8th.
I think asking Dominguez to pitch on back to back days is probably worse than asking him to pitch a 2nd inning. Prior to tonight's game, he had a .270/.413/.405 opposition batting line with a 1.88 K/BB ratio in appearances with 0 days rest versus an overall line of .163/.246/.270 and a 4.13 K/BB ratio. That's a huge difference in performance (with the caveat that since he's a reliever, everything we talk about is small sample sizes).
