This is not possible in LyX 2.0.x. LyX 2.1 provides a very flexible argument framework. LyX 2.1 will be released in a couple of months. You can use the development version (pretty stable) now alongside your current version on Ubuntu very easily with the LyX PPA.
To accomplish what you want to do in LyX 2.1, use the following:
Style Classify Category FrontMatter LatexType Command LatexName refine Argument 1 Mandatory 1 LabelString "FirstMandatoryArg" EndArgument Argument 2 Mandatory 1 LabelString "SecondMandatoryArg" EndArgumentEndAttached is a screenshot that shows you how to use it. Note that the items are greyed out in the Insert menu because I've already inserted them. Note also that the order doesn't matter (thus A C B will still be {A}{B}{C}). You can see this in the LaTeX preview at the bottom of the screenshot.
Image may be NSFW.
Clik here to view.
