The resin in a di unit can be mixed or used in seporate filters. If you check the polarity of water after your ro unit, the water will have a charge (positive or negative) once you determine the charge, you will know how much resin to use. Typically for sake of ease, both resins are equal in a di unit...equal parts of both resins are NOT needed. Only about half of one of the resins usually being used will suffice. In other words, if you are using equal amounts of both resins, you are wasting one of the resins. Here in the US any decent water company can help cosrt out the ratio by a simple water polarity test. That, in and of itself, will reduce your resin cost by at least 25 %. Also, if you are using a mixed bed (single filter full of resin) you cna purchase another filter for resin and seporate the resins resulting in a longer resin life.
However, if your unit sets for several days at the time, the resin will loose its filtering life. Always run a gallon or two a day through your unit to extend the life of resins.