We love Fuzzi Bunz pockets and use them full-time for our little boy. He is now 19 months old and still easily fits a medium, which he started in at 4 months old. I know of quite a lot of toddlers that still wear medium until they toilet train.
It sounds like both your boys would fit a medium really well.
As I liked the Fuzzi Bunz nappy system so much we are now managing their newborn hire kits. They only fit up until 5 months old or so. We also sell the medium size after mums have tried out the hire kit, so I can tell you about the cost.
A good number for a fulltime set is 12 with some extra inserts and boosters. That would cost about $380. With two boys in the same size, you probably get away with having 20-22 naps and extra inserts. The resale of Fuzzi Bunz on TradeMe is very high, with most of them selling between $20-$25. Taking that into account you could probably only have spent $150-$250 for the year for both your boys. Okay, and you add some extra washing powder to that equation as well!
If you are interested to try some out just PM me. We always have a few mediums for mums to trial. We will only charge you the postage cost.

Nathalie