Using Merge Tool - only two at a time as stated, however sometimes it can be useful to use the copy function, if for instance 4 surfaces are connected you can select all 4, and use the copy / paste option
This creates a complete whole surface which joins the 4 individual surfaces, this helps in certain instances
> This creates a complete whole surface which joins the 4 individual surfaces
Unless I'm missing something it creates a single feature containing multiple surfaces / quilts but it does not join surfaces, e.g. create 2 sided edges where there were 1 sided edges.
Unless I am missing something totally, I just tried that copy multiple surfaces and paste in one command and it wont allow the selection of surfaces that are not merged.