If you are upgrading the size of the CB panel then you should be sure that the incoming service can handle the amperage. With a 100A service, the incoming wires should be at least a #2 size or even #0 depending on the distance of the wire.

As others have mentioned, be sure that you have 220V available at the barn. You could have one 40A wire feeding the current box and then it is just split to the two 20A circuits.

If you have any questions, you should contact a local electrician and have him/her give you a hand with it.

good luck