This needs a slightly elaborate filtergraph to do as described.
0 is video '1' and
1 is video '2'.
DUR0 should be replaced with duration of video '1' and
D with duration of the effect.
A curtain effect, like what Paul described, can be done like this:
S is start time in seconds. (You may want/need to adjust the timestamps of the top video 'A' beforehand using
D is duration of effect. These two variables should be replaced with the required numeral values.
I can't test either of these for a few days.