Q:

How can a destination gate glow when being dialed?

In episode The Tok'ra II. Sam, Her Father and the ...Lover Boy was leaving the Tok'ra under attack planet. They are now in front of the Gate, then the Stargate lit up (as if being dialed as destination). Sam says, they are dialing here.. and also said, not if we dial Earth first.
She dialed earth but unsure who dialed first, if their current gate is a destination or the entry point. Her father stated that they dont have a choice. Lucky day.. they arrived to Earth!
Question is: How can a destination gate lit up as if being dialled from another source, when the address is still incomplete. 

A:

I always assumed that the caller dials the "number" and afterwards, the numbers are sent to the gate network, one by one. Otherwise, as you very well put it, it doens't make sense.
The other posibility, is that while the person is dialing, all gates that match the unfinished address start to light up and turn off when they realize it's they're not the destination, but that's never been seen (at least not on the show).

A:

There is a subspace connection between gates that happens in real time, this is why when dialing the sequence is visible simultaneously. (There is a bit of a plot hole here since there is no way for the gate to know which it is dialing until the end of its sequence, but that can be hand-waved away as we are never shown other gates with similar addresses at the same time so it is possible that they also light up until the address is no longer possible).

