I have an old 10-speed with take-apart fittings on it, that lets you
separate the frame
into two halves. It is (IIRC) late '70s vintage. In that condition it
will fit in the back seat of
a cessna (wheels in place), but pretty well fills up the space. I doubt
that you'd get two
of them in there. I haven't seen anything like that advertised in a
long time. However,
there are numerous types of folding bikes available (check eBay).

If you do carry a bike in an airplane, take care to protect the
upholstery from the oil
and grime on the chain and sprockets. An oversize garbage bag will
serve this purpose.

David Johnson