I keep my extra scobys in what I call my Scoby Hotel. It's a secondary gallon glass container that I put my extras in. I switch out the Kombucha every 1-4 weeks, depending on how many scobys I have in there, If I'm trying to achieve extra Kombucha, or make Kombucha Vinegar. You set it up just like you would a batch of Kombucha only it holds all your extras. The benefits of storing your extras this way are they are in a separate container from your main brew so you always have an extra one on hand should catastrophe strike and you loose a batch (and scoby) to mold/flies/etc. Since this container is kept on the counter you don't have to worry about the bacteria and yeast becoming sluggish like you do if you store in the fridge. Plus if you replace the kombucha with new tea when you bottle your normal batch its a way to get even more home brew while achieving a nice habitat for the scobies.
You can also leave your extras in with your regular brew, however some risks come with this if you are brewing in a single container. If you happen to have a batch that goes bad, becomes infected by flies or mold you loose all of your scobies. I only recommend this approach if you have a couple of containers and can spread your extras out evenly between them. Just remember to keep a check on the brewing Kombucha because with the increase of bacteria and yeast it may get to the point you prefer your Kombucha a few days earlier than if it just had one scoby in it.
If you are needing to take a short break or want to store your extras in the fridge, make sweet tea to feed the scoby, put it in a jar and place a lid on it. It will stay in the liquid for some time as the cold slows down the metabolism of the scoby and it takes it longer to then convert the sugar and tea to Kombucha. Please note if you choose to store your extras in the fridge you should check on them at least once a week and add more tea if it seems low or less sweet. Also the first round of Kombucha may take longer to brew after your Scoby has been in cold storage. It can take a couple batches to speed the metabolism of the yeast and bacteria back up, so don't be alarmed if your initial batch afterwards is a bit slower than you are use to.