Seperate cation and anion resins are used if you are going to use acid and lye to recharge the resin. To me its not worth the effort and hazardous chemicals to do this. You use each resin in a seperate canister and recharge them seperately but normal mixed bed resins are not 50%/50% blends so with seperate resins you can end up with pH problems if not used in the correct amounts. Also recharging your own resins at home will never get you a 100% recharge, it takes heat and other conditions to do it properly.
Semiconductor or nuclear grade mixed bed resins are the most popular. My local reef club buys a cubic foot or two at a time and reseals it in "seal a meal" bags for individual use. This is a pretty good option for most groups as you get the best pricing if you can use that much. For an individual you need to remember the shelf life of DI resin is about 6 months in the best of conditions (sealed, climate controlled, dark and damp) so don't buy more than you can use in that amount of time.