There's no fixed limit. The closer together the conductors are, the more
they'll couple. The coupling has two major effects. First is that the
resonant length of one wire will depend on the lengths of the others; in
particular, shorter ones will be affected by longer ones. So tuning will
be tricky and somewhat interactive. The other is that the antennas will
be more narrowbanded than when the wires are separated farther. So you
get to choose whether to space them closer and have the effects of
increased coupling, or space them farther apart and have them be more
independent.

Roy Lewallen, W7EL

funkbastler wrote:
If, on each side of the feed point, a single conductor runs out
to point X, with individual conductors onward from X for each
band of interest, how far from the feedpoint can X be?

(like this) /10 meters
/
FP----------------X------15 meters
\
\
\
\20 meters