You will not be able to do it with only one I.P. address without changing your ports around.
The router needs the ports to be “forwarded” to the internal I.P. address of the Quintums.
If both use the same ports, the router will not know who is who.
I gues in theory it could be done, but one of the quintums will have to go for non-standar ports for ras and voice packets.
Not easy unless you control the other end that one of those quintums talk to.
If you are on the U.S., will be better to get a second ADSL, over one of the lines used for the Tenor.
Will save you troubles and give you some backup.