The Radio Information Board

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

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

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