Can more than one Mining program/machine use the same worker?
Yes
[EDIT]
I've a lot of CPU miners agrupated under only 1 user, cause make me easy modify settings for all in a while
I was thinking that this would help people using multiple slow machines hold on to their shares over time. Unless all workers benefit from each other's time since last contribution made, in which case the result would be the same.
Yep, I'm pretty sure all workers are scored cumulatively. So all your workers are contributing to the same pool of work, so it doesn't matter whether you have one worker with 10 machines attached, or 10 workers with 1 machine attached, you should get essentially the same payout regardless.