Some notes on my RTTY setup. The first method for splitting the rx is used on the SO2R setup so you can decode the main VFO on both radios. The second method is used on all the stations for m/m operation to decode both VFO's on the FT-1000mp's.
With mmtty you can set up separate decoders on the left and right channels of a stereo sound card. All you need to do is get a stereo Y cable or connector that plugs into your sound card line-in jack. What I use is a Y cable, I slit the outer jacket of the cable open and expose the inside wires. On one side I clip the wire that goes to the ring, on the other side I cut both wires and connect the one that comes from the tip input to the ring output. So now the tip of one side goes to the tip of the output, and the tip of the other side goes to the ring output... so now one radio is in the left channel and the other is in the right. set up 2 instances of mmtty and point one at each channel and away you go.
The output is even easier, just use the same type of splitter unmodified, run the same audio to both radios. then set up your ptt or tx via cat command to only the selected radio is keyed.
Its even simpler on an mp to use the sub rx, just use a stereo cable from the line-out on the back of the mp straight to the line-in on the sound card, set up 2 mmtty's like above and you can decode both vfo's at once... of course you can't rx while tx in that arrangement since the sub rx is muted.