Doors and Windows - double windows

I'd like to install several pairs of windows side by side and am not sure how to go about doing it. The house is an old timberframe and I'll be starting the process from frame up, so I can set the framing up any way i'd like. I've removed the brick molding and will replace w/ 5/4 cedar. What's the best way to do this, should I join the windows together and then install or should I install seperatley. Is there a need to put a 2x between the windows ? Lastly what would be the best way of joining the sills together so I minimize the potential for water damage. Thanks.

it depends upon what kind of windows you are getting. Some can either be ordered mulled together, others can be mulled together if you have the correct piece. No need for a 2x between unless you need/want to space the windows apart for some reason.

You mention joining sills together- are you putting in some old windows? Hardly any windows come with sills anymore.

Thanks:
These are new old-style windows made to match the rest of the windows (single pane, true divided light) in my house that were replaced by the previous owner about 5 years ago. They do have sills, which i'll butt end-to-end with some caulking between them.