In your particular example (break into hideout) the following comes into mind:
- Staking out the place to gather information on what their defenses are like: security procedures, guards, their armaments, do they look like they know their stuff, are the local bluecoats on their payroll etc… This could be done near the site physically observing them, or maybe in the local gin houses talking to people who know the gang and place or maybe addressing the bluecoat contact or another gang
- Are there any weak links in the gang? People who are totally addicted to drugs or something else, people who are gullible or easily bribed or have other weaknesses. Surely someone will know. Need to gather info on that.
In general gather information is used exactly when you don’t know lot of stuff. The roll will decide how good/accurate info you have. Also remember that the GM is not the only one who writes the fiction. If a player wants to gather info on said gang members and hopes to find someone who is totally easy to shift his loyalties and he rolls well then there absolutely should be someone like that. Unless the fiction dictates that these gang members are totally loyal, then there probably won’t be. Or will there? Up to the table.
As for the engagement the group does not have to start together. The engagement roll is simply used to set the first scene for the score. If there are two groups you really do not need another engagement roll: just switch the “spotlight” when appropriate to the other group and explain the situation to them and the pos/eff will come from that.
In your example the observing player simply comes in on the fiction at some point when appropriate. Remember that it is a game where everyone should have their time to shine in the spotlight.
“So Eve, you are observing the rest of the group who are disguised as prisoners. Apparently something has gone a bit wrong since the guards are suspecting something and not letting them in. (They got 4/5 their SWAY and guards are not convinced, GM gave limited effect as consequence) And on top of that you notice your local bluecoat, Dermot, who is walking towards the group now. He will definitely know your guys… what do you do?”
Check out Teamwork on p. 134 for inspiration on how splitting the group could be done using set up actions and so forth.
Also check out “linked plans” on p. 131, though that is for more of a long term splitting of the group.