Not quite correct Frank.
ISDN is a signalling protocol using an E1( 2Mb) or T1 (?Mb) link as the carrier. As such it has a send link and a receive link. At the B channel level this translates to 2 seperate 64kb channels per conversation, one for send (A>B) and one for receive (B>A)
It is really only at the LAN level that the send and receive packets are on the same transmission media and the bandwidth capacity per conversation MUST be doubled to calculate the b/w of the LAN.
Regardless of the technology used for transmision between nodes the transmission link usually consists of seperate send and receive channels. THe only exception I can think of is when the link is another 802.? connection, ie ethernet cable.