Learn English – Synonym of ‘Run’ or ‘Operate’ in the context of a computer programme

The most appropriate term I can think of is composite, used either as a noun or adjective. NOAD provides some excellent accompanying examples:

composite

(adjective) made up of several parts or elements:

• (esp. of a constructional material) made up of recognizable constituents:

  a new composite material—a blend of plastic and ceramic resins.

(noun) a thing made up of several parts or elements:

• a composite constructional material:

  the next decade may well see the introduction of more designer polymers and composites

Since composite refers to several elements, not just two, you could use it in your example this way:

... between the two different sites in the composite blend

using the word two to specify how many elements there are.

The answer provided by Kris, however, is more technically appropriate for the specific example you mention. However, combining this with your suggestion, binary composite is certainly an excellent alternative. Binary mixture is also technically correct, as all composites, amalgams, blends, etc, are simply mixtures.

I think that the best alternative, in this case, is “made optional”. The only (putative) fault it has is that of not being a single word. I think it has the advantage of being easier to understand — simpler. (If you count the space as a letter, it is only one letter longer.)