Yes, certainly you can use two of the 3 channel decoders. Just leave the "channel 6" without connection and give the next controller the startadress #6.
The only thing you have to consider: the specifics of DMX says, that without a repeater you only can attach 32 devices. In reality this depends on your outputinterface and your cables. Using two 3 channel decoders instead of one 5 channel allows you to use only 16 of your rgbwa-devices before you need an active repeater (or splitter).
If available, you could also use e.g. 10 channel controllers and leave 5 of them without connection. All of those mixes are not relevant for the function of DMX itself.