The reason there is waste with one and not the other is because they are two different ways of purifying water . In simple terms the DI is dealing with all the water passing through, purifying it on the way past the beads. The RO on the other hand is separating the water, all the good stuff out one pipe all the rubbish out the other. The waste water has the impurities that would normally be in the product water as well as it's own.
The waste water is much higher TDS (total dissolved solids) than just tap water, so you wouldn't want it going anywhere near your DI. It would be better just to put the water straight from the tap through the DI without going through the RO first.
RO is a much cheaper way of dealing with the water than DI, it is just unfortunate that it is also a much slower process.
Peter
