Basic rate ISDN has 2 B channels. It is I believe usually possible to use them independently.
You will need:-
Not to be using Multilink PPP so that the first call uses both channels.
Not to use the physical interfaces for the termination point of the calls.
Maybe you can use dialer interfaces, in that case you will need two of them, or maybe you have to use virtual interfaces or virtual template interfaces.
In any of the above cases the physical interaces are dynamically bound to some sort of virtual interface when the call comes in.
Post configs. Sanitised as required.
sh tech displays a partially sanitised config in that there are no passwords but your phone numbers and private IP addresses will still be in it.