I've done exactly what Herb suggested: hung a sloper for 80M off the side of my 160M shunt-fed tower. Neither antenna knows the other is there and both work pretty well - meaning I'm satisfied with what I get out of them.
Interesting. I have tried verticals, slopers, inverted V's and other 80 meter antennas on or near my 160 meter tower with little success. The impedance of any such antenna seems to be severely altered by the nearby tower. I suspect it depends on the electrical length of the tower. Mine is close to an electrical half wave on 80 - 100 feet of Rohn 25 with a 7 element 6 meter yagi, approximately 30 foot boom sitting at 103 feet.
At one point in time the 160 meter shunt feed could be made to provide an excellent match on 80 simply by changing the series capacitance. However, after I cut down several nearby trees that no longer works.
73, Paul N1BUG _________________ Topband Reflector