I'd have done a 50% waterchange if your ammonia was that high.
You want as much bacteria in the QT
as possible. I'd seed the 2nd sponge for at least 10 days, then combine the 2 together.
In the meantime, keep up with your daily waterchanges and be very conscientious with feeding.
Remember, underfeed rather than overfeed - a lot of marine fish go for days (sometimes even weeks!) without food in the wild, so maybe even cut down on feeding (to a degree) to help you.
Bit of an ar*e, but it'll be well worth it in the long run!