TK-790/890 crossband setup help

[KF6OWC]

OK so I though I would build up an interface so I could crossband my 790 and 890. No problem (I thought) I got a drawing of an interface I used for some motorola radios that had worked for me before. RX and TX audio, and PTT aren't a problem, kenwood unlike the motorola the COR goes high (open) instead of low (ground for the PTT). My plan was to feed the COR thur a 4N32 optio coupler to the PTT. I would then switch the 4N32 with the AUX A output the goes Low when the AUX A button is pushed. I am dooing all of this using the DB25 connector on the back of the radio. Any suggestions on how to get a ground for the PTT of the other radio when a signal is recieved on the first radio, and vise versa? maybe some sort of NPN switch? Remeber that the COR is low without a signal, and goes high when it recievs one.
Thank you Jim

[cms1126]

if you buy the dual band kit for it from kenwood you can set them up in the software to have like 5 cross band channels

[KF6OWC]

true how ever this is two seperate radios, with the KRK-7DB you have to preassign up to 20 channel as preset crossband repeat pairs. I travel and am in frequently in different areas with many more frequency than that. plus I often listen simultanesouly to both radios, In fact I have a 790 (CDF) 890 and 690H, plus a scanner for trunked systems. I was hoping to find that some one had already figured out a interfave using the DB25 on the back of the radio.
TNX Jim