actually you are wrong here, it is possible.You can't. At least not 1.6 and 1.7. Forge changed a lot with 1.7 which requires a custom bungeecord build and an up to date forge version. From what I know you can't use the custom bungeecord version with 1.6 forge servers.
The question here was if you could connect 1.6 and 1.7 servers to one bungeecord instance. Not if you can use bungeecord with 1.7 servers. I'm not aware of any way without custom modification that would allow you to use one bungeecord instance for 1.6 and 1.7. If you know more, I would be happy to hear it.actually you are wrong here, it is possible.
My friend was the one who fixed bungeecord 1.7.10 to work with forge, his name is dualspiral, anyways it was fixed all you need to do is make sure your cauldron versions are using forge 1230 or higher, dw20 uses a forge version higher then 1230 so its possible, aslong as all modpacks use forge 1230 or higher it will work.