Topband: 160m/80m duoband vertical question

[Jim Garland]

My 160m vertical is 79ft high (wire running up a 26m Spiderbeam fiberglass
pole), with a capacity hat. Is it workable to put an 80m 1/4 wave wire on
the same mast join both antennas at the base, in the same way people make
multiband fan dipoles? The idea would be that when operationg on 80m, the
160m antenna would present a high Z, and vice-versa when operating on 160m.
I don't have a feeling for how much interaction there would be between the
two wires, separated by only a couple of inches. Another option would be to
electrically isolate the wires and select one or the other with a relay.  If
possible, I'd like to avoid matching networks and complicated switching
arrangements, because the antenna is 700 ft from my station. Any advice is
most welcome.

73,

Jim W8ZR