On board signal termination
Forgive my bad drawing. So what's happening is  that you have the opportunity for multiple reflection paths.

In your original setup, if CH_A is active, you have your signal going doing A, and when it gets to ABC node, you have a bit of your signal going to B, and then to C because your impedance halves at this point since you have Zo going to B and Zo going down C (assuming your traces are the same thickness - hard to tell). When each of those reaches the end, they reflect back and it goes all over the place because once you get back to ABC, your impedance changes. This is your ringing. Will this degrade your edge - probably not. A simulation would probably be your best bet. 

but if you connect your trace like in D, then there is only one reflection path (which is good!). 

so if you have a series termination for CH_A and CH_B, the signal will die out once it reaches the driver.