Bill Cherepy wrote:
"---can have a full wave 49 meter loop inside a full wave 80 meter
loop---?"

Bill Orr, W6SAI wrote a book, "All About Cubical Quad Antennas". One
topic is "Concentric Quad Antennas" starting on page 49. Bill Orr says:
"Interaction between the antennas of a two-band Quad is quite low."
Orr suggests separate feedlines for each antenna and making feedline
lengths odd multiples of a 1/4-wave so that the open-circuit of the
unused feedline will appear as a short-circuit at its antennas resonant
frequency at its feedpoint. Thus the unused assembly won`t likely
respond much at other frequencies.
Of course, antennas have a second resonance at about double their first
resonant frequency. Having the open end of the unused feedline
available, one could deliberately detune the second antenna to reduce
interaction.

Best regards, Richard Harrison, KB5WZI