You might want to wait 1-2 more feedings before going double. I did end up with 1 regurge, so I'm now thinking better to wait just a bit longer. It's hard to say, I only had trouble with one and the others were ok.
I have one who is way behind on weight too, she was 3 months old when I got her and she ate. I need to start slitting pinks I guess. Maybe it would help my non-feeders I'm working with to eat mice.