The M60 should spawn just before the gauntlet.
Not everytime. I've had a couple playthroughs where it never spawned. I've had one where it spawned in the first store (this was an expert play through so I don't know if that makes a difference or not). It's unpredictable.
And so far I've NEVER seen more than one spawn at the same time. But it is cool to imagine the survivors with two of them.