So you want a Custom Firmware for the DSTwo+ itself?
Afraid that's not possible.
The DSTwo+ is a DS Flashcart with a gateway clone inside it.
The DSTwo part is locked to running in the systems DS mode. It's not even possible to run the DSTwo in DSi mode, let alone 3DS mode. The code is just totally different between the 3DS and DS, so it's simply not physically possible.
If you want something that can use the full power of the Nintendo 3DS, you need to be running 3DS homebrew.
Now if you wanted a Custom Firmware for the 3DS, I suggest looking in the 3DS's Guides Section.
I presume that you have the DSTwo+ working AS a gateway clone with one of their older versions, which means it shouldn't be to big a jump to switch to CFW, presumably rxTools.