Transitions will always stop the outgoing video and show a delay of the incoming video.
The best way is to combine the two videos using a video editing application but the following works well using builds
place the two video files and a shape filled with black that fills the screen on the same side
create an in build using a dissolve on the shape
create an out build using a dissolve on the shape
drag the items in the build order to this
set video 2 to Appear
dont include a build for video 1 as it is to play automatically when the slide is played