How often is this going to happen? I have the perfect solution for you:
Buy a used E-Mu 1616m breakout box and run it into your 1212m soundcard via CAT5 cable. The 1616m is designed to work with the 1212m without a hitch. Just disable the analog input/output sister card that you have in the computer right now, and only use the main card.
The thing is this: Standing alone, the 1212m is only a very good bang-for-the-buck soundcard with better AD/DA conversion than most things in its price range. If you can deal with the learning curve associated with PatchMix, the software is very useful. But, you need to have separate preamps and a separate headphone amp, as well as a way for the single pair of balanced outputs to feed both your monitors and a headphone amp. This means buying more equipment than your budget allows if you buy it all separately. If you go this route, a good budget headphone amp that will work great for your situation is the PreSonus HP4. You feed the balanced lines out of the 1212m into the HP4, then back out of that into your monitors. These are pretty inexpensive. However, you are not going to have a lot of money left over for decent preamps.
If you get the 1616m breakout box, you get 2 OK (bland but OK) preamps, a headphone amp, and all the ins and outs you could ever possibly want.
I just looked on eBay and saw a used 1616m on sale for $250. It is the kind that has a card for a laptop, but you aren't going to need anything other than the 1616m box itself and some CAT5 cable.