If you can fit 34' on EACH side of the antenna, then you have space for a 40m dipole.
Start with that. Then make a 20m element and add that to the same feedpoint.
The last step is to put a loading coil on the end of the 20m element with an extension
wire so the total length is about the same as the 40m dipole: adjust the coil size until
that antenna is resonant on 80m.
Three bands with coax feed on a common feedpoint.
AE5JU wrote an article on how he implemented this idea: http://www.hamuniverse.com/ae5jufielddayantenna.html
It's on hamuniverse.com, so don't take it as gospel, but the general approach should
work. You don't have to run the wires at right angles - you can string them between
the two trees with one hanging under the other, and you can adjust the loading coil
to accommodate various lengths for the 20m/80m wire.