Ron,
It's mainly a matter of convenience, or maybe you only
have enough feed line for one feed line.
Depending on 'how' you use the connected-together antennas,
only for use on the intended bands they are cut for, or as
multiband antennas, it might be simpler to use seperate feed
lines for each.
As for the 'best' way of connecting two or more antennas
for use on a single feed line, attach them to whatever you're
using for a center insulator, the second antenna just like the
first one. There are a number of 'mechanical' ways of doing
that and the 'best' is which ever is easiest for you (or how
ever you know how to do it). As you'd expect, the connections
should be strong enough, light weight enough, and sort of
'pretty' enough to satisfy the antenna's requirements and any
'significant other' that's around (wife, neighbors, landlord,
etc.).
Using seperate feed lines and an antenna switch makes the
process of tuning the antennas a bit 'easier', probably won't
take as long. Which ~may~ be the only difference depending on
how/where you erect the antennas.
Of course, using antennas that are high, widely seperated,
and
fed seperately is always nice! If you have the room, the
required
feed lines, a switch, the time to do it, and so on (I wish!)...
'Doc