For just two bands, adapting K6MHE's 20m/40m dual bander for 40m/80m might work well:
I came up with the same circuit years ago, only to see it published by someone else (W1FB?
W7EL?) shortly afterwards. The SWR bandwidth on 80m is narrower than it would be otherwise,
but the circuit works well. I tuned it up at low power then replaced the variable capacitors with
Using two quarter wave elements should also work - there may be some tuning interaction,
but once you get them adjusted it shouldn't need any further attention. You can always add
small series coils at the base to lower the resonant frequency if needed. (Or add fixed
capacitors to raise the resonant frequency, but if you don't have capacitors you can at least
cut the wires a bit short and wind your own coils.) This might be a good option if you can't
get them both to resonate where you want them by triming the wires.
Balun??? A common mode choke (1 : 1 current balun) in the feedline perhaps, but please not
one of those 4 : 1 baluns that are so often mis-applied for end-fed antennas.