I've got an HT and it's got a basic dual band 144 MHz/440 MHz antenna.
It's about a meter long, which would be correct for the 144 MHz band.

What makes this antenna different from a 144 MHz (only) antenna?

Are there typically two conductors inside, with one 1 meter and one 35
cm? Or are the antennas typically just identical to a 144 MHz (only)
antenna, and it just works at 3/4 wavelength at 70 cm?

As I understand it, a 3/4 wavelength whip should work reasonably well,
being resonant and having a similar impedance to a 1/4 wavelength
whip. Is this correct?

To be more to the point -- I've got a Kenwood D700A in my car. It has
a built in duplexer, and one antenna jack. I've hooked it up to a 2
meter antenna on my roof, and it seems to work fine, both on 2m and
70cm. But I'm wondering if I'm risking ruining it ...

(I haven't put a SWR meter on it yet. I probably should.)

And what's so magic about 5/8 wave antennas? It's not even resonant,
so obviously a matching network of some sort will be needed, but why
5/8? Why not 11/16? Or 3/4? Or something else?

--
Doug McLaren, , AD5RH
`What's the use of happiness? It can't buy you money.' -Henny Youngman