Agreed with KathyP - get a mite count on BOTH colonies first by either 72-hour drop onto a sticky board or a sugar/ether roll. The sticky board drop is probably easiest for a new beek.
Then, you will know how many mites you have and can treat one or both colonies with the product/methods of your choice. Just don't go for the Apistan or CheckMite...many many areas now have strong resistance in the mites to these products, so they won't do much good.
Then, after you treat, remember to do another mite count on both colonies (even if you only treat one). Get in the habit of keeping count of your mites all the time, not just when you see a problem. This will give you the information you need to track the mite population and know when it's just a pest and when it's really a problem.