Cross Dissolve

1. on your first video place a keyframe with an "opacity" of "0" and where you want your video to start dissolving place a keyframe with a "opacity" of "100"
2. place your 2nd video on top of where your first keyframe starts to dissolve
3. for your second video place the keyframe at the start with an "opacity" of "0" and then your second keyframe where the 1st video ends with an "opacity" of "100"