You could create a scene with either a black color square or a 10 second filler video and use Advanced Scene Switcher to do a 10 second transition to it from scene 1, wait 10 seconds and then do a 10 second transition from it onto scene 2.
Another approach would be to use the scene switcher to do a 10 second transition from scene 1 to scene 2 immediately followed by a 10 second switch back to scene 1 followed by a 10 second switch to scene 2.
There is also a plugin from Exeldro called Transition Table. I don't know that it allows longer transitions, but its probably worth investigating.