Brian is correct - you're bridging two groups, so you're actually using 3 groups, and yes, a TM751 will only control ONE HouseCode.
I think a problem is that while activating a camera within one group turns off other cameras, it only turns off the cameras within the same group. And the groups are as Brian indicated, 1-4, 5-8, 9-12, and 13-16. Just using 4 consecutive numbers does not create a group. If you change C3 & C4 to C7 & C8, you'll have those four in a single group and activating one of them "should" turn off the other 3. - But that will have no affect on the "F" cameras. And then, if one "F" is on at the same time a "C" camera is, whichever has the strongest signal will be seen - and you'll probably have a ghost image mixed in. And selecting an "F" camera won't turn off the "C" group.
You can test it by selecting a "C" camera, then sending an OFF to whichever "F" camera is interfering.
OR - you could use 2 receivers and set them (and their cameras) to different channels. - (But you still need to get all 4 "C" cameras into the same group)