I bought two fantail goldfish recently (within the past couple days), and it appears that one of them has Ich.  I just found out about the disease today, and so my other, healthy goldfish has been swimming around with my Ich infested goldfish not only for the past few days, but also in the aquarium from where I bought the fish.

It says to quarantine the infested fish for 10-14 days of treatment, but since the other fish has had extended exposure to the other one, would it be wise to leave them together and treat them both at the same time, to ensure that any parasites won't infect my healthy fish?  Or must I absolutely separate them at all costs during treatment?

I'm using QuICK Cure by Aquarium Products, if that makes any difference.  I just wanted to get some input, because I'm pretty new to fish care and don't want to do anything to hurt my lovely fantails!