The vertical antenna I described on the 160M antenna page worked well on both 160 and 80. But there were some challenges.
To start with, I could not operate on both 160 and 80 at the same time. Well, they were the same antenna. And then I found that the vacuum relay at the top of the antenna self destructed every 5 or 6 months. And the antenna is big and heavy enough to be a pain to lower and replace the relay - which, BTW, is not cheap.
So, one day I realized I had the disassembled 43 foot vertical and a radial field just not doing anything. I put up the vertical back, added a roughly 25 ft horizontal wire, added a coax line, and presto, I had a dedicated 80 meter inverted L. The antenna is back in the forest out of sight but that doesn't hurt it at all.