There is a much easier answer to making a 160m inverted-L which can be
instantly changeable switchable to an 80m vertical - no relays, no worries.
Simply connect a 160m quarter wave inverted-L made of wire in parallel with
your 80m vertical, taking care to space the vertical sections of each
antenna about one foot apart for minimum interaction. I got this spacing
from one of the Garys - either K9AY or KD9SV - on the reflector some time
ago and it works like a charm.
The antennas each retain their normal bandwidth and impedance.
I use a similar technique on my 160m/40m parallel inverted vee dipoles,
simply spacing them about six inches apart. Again, works great!