What to do with the space for window counterweights?

[Curt Bixel]

I have a property I am rehabbing.  The siding and windows are in such condition that it simply makes sense to go with full, new construction type windows with the integral nailing fins and J-channel.  Then finish the whole house with reasonable quality vinyl siding.  The windows are all pretty much 30 inches wide and 67 inches tall.  I have included a picture of the windows below.

I am curious what to do with the central column between these two windows.  If I go with two 30 inch wide windows, what happens to the 4 inch space between the windows currently being used for the counterweights.  If I leave it there, then I will have two window with J-channel, that are about two inches apart, leaving a tiny space between them.  Do people usually put siding between these two windows?  

Is another option to order two 32 inch windows and then fasten them together?  If so, do I buy the two double hung windows already attached together at the factory, or is this something I do myself on the job site?