You can attach them, but it's usually not possible to do so and maintain tangency in a patch modeling situation.

If you're just trying to attach two surfaces, that's not a problem.

My site has a couple tutorials going over this kind of thing. For just attaching two surfaces, check out "Attaching NURBS Surfaces Cleanly."

The main things you need to look for in attaching two nurbs surfaces together is

-matching connecting surface direction (U or V)
-matching number of spans

Normals can also play a part.

It's also important that nurbs surfaces be rebuilt. Not necessarily to change their spans, but just to get rid of 'surface background noise.'

Once you have both surfaces attached like you want, be sure to delete history, as when you try to move it, it will have the history of the seperate pieces and will move accordingly.

Hope some of that helps.