Geomipmapping

With skirts it shouldn't matter what the level difference is. The skirt geometry should extend down far enough to handle any possible gaps in the terrain. I've never actually implemented it, but I know that is the basic idea behind skirts.

If you wish to connect your two patches together and fill the gaps, there are several techniques.

The simplest one is the "skirt", where you just add a skirt, really, like a polygon belt that goes down enough so that gaps will be filled. You might see some steps on the sides, though.

A more complicated technique is to add triangles on the lowest resolution patch to fill the gaps exactly. Depending on your courage in implementing this, you can either make sure that the difference in resolution between 2 adjacent patches is not greater than 2, or go for a more complicated re-grid to handle all the cases. These complex re-grids can often make use of fan polygons.

And if, like me, you get bored in implementing your own landscape engine, you can use Demeter (http://tbgsoftware.com).

There are plenty of ressources on the subject, out there. Check there: http://vterrain.org/LOD/Papers/index.html. This is a gold mine!