Grayson,
I think that there's a very simple way to accomplish what you want to.
If you get DCS, simply connect the TIU in the manner described(in series with the track wire connections). Then, take your Legacy base, and connect it to the outer rail, in the manner described.
Doing this, you can run Legacy engines with the CAB-2, and they won't know or care that you have DCS. You can run PS-2 engines with the DCS handheld, and they won't know or care that you have Legacy.
If you care to, you can control Legacy through DCS. Its usefulness is limited, however, and you lose all of the Legacy features like quillable whistle, etc. Basically, controlling Legacy through DCS makes it function like TMCC1, and only in a somewhat limited capacity at that.
