I think that gness is near the mark. They could be observing/checking on someone, and two cars would be more noticeable than one car. Local authorities sometimes check on people who are claiming the community charge discount for one adult by watching the house and noting who goes out and who goes in.
One could be just a learner driver, they probably meet on your street, do a few routes that may come up in an upcoming driving test and return to collect the fully qualified drivers car after some time
Now I'm curious
I wouldn't have thought they were engaged in any criminal activity
If that was the case I wouldn't choose our road to meet up given that it's a quiet cul-de-sac and where it's very likely that someone would notice them meeting as I have done
I'm sure I'm not the only one who have noticed them
I'll ask my neighbour when I see her next - who lives opposite to where they park if she has noticed them
I suspect your close is just a convenient half-way place to park one car off a main road where it is felt to be safe so that the other car can be used for the journey to save fuel/parking costs etc.
I'm in a cul-de-sac Baz and when a stranger or strangers park in our way I usually go out and ask are they lost or looking for someone, it usually ends up in a conversation, by time they're gone, I know them and all belonging to them, that's me being rightly nosey