I wouldn't recommend putting another fish in a two gallon tank, it's too small really especially for something like a shubunkin which should really be kept in an outdoor pond. They might look like they have plenty of room but they produce a lot of waste and another goldfish on top of the one she already has will produce even more waste. She will probably find the water needs changing more often and it could lead to health problems for the fish. Sorry to put a downer on things but I learnt from experience!