You can get KNX group addresses to sync to C-Bus group addresses and vice versa through Comfort
KNX and Cbus group addresses both update Comfort counters, so you can use this mechanism to synchronise
Say Cbus group address 10 and KNX 0/0/1 are to be synched together
KNX 0/0/1 can be assigned to a Counter, say Counter 1. When 0/0/1 1 bit telegram is received it updates the counter 1 value to 0 for off, 255 for on and activates a Counter Response. This counter response can be programmed to send a telegram to Cbus group address 10, eg If counter value = 0 send Cbus group 10 off, else send Group 10 On. This makes Cbus group 10 follow the KNX group 0/0/1
To go the other way, Cbus group address 10 is mapped to counter 10, so you can use the same method to send telegrams to KNX 0/0/1 based on the value in counter 10, which is 0 for off and 255 for on
Not simple but what you are trying to do is not simple either. But this method is proven because we have this scenario at our office. We have both Cbus and KNX switches and actuators, and they appear to be part of the same system
Let me know if you need more help or examples
Also you should have a UCM/KNX which is the 2nd generation KMX interface, see
http://www.comfortforums.com/forum12/1533.html
The UCM/KNX is much more powerful and easy to use compared to UCM/EIB
I have moved this to the UCM/Cbus forum though it could have gone to the KNX forum as well