If the windows tegether are narrower than about 72" then you will have to have a new header for a 6' double door.
Figure a couple of 2by8s x 8' long or a couple of 2by10s for a stronger header with something like 1/2" plywood or drywall as a spacer and some studs.
Check around on prices. I think I saw where home cheapo installs then for $200 or $300. It could be more depending upon what needs to be done to match the interior and exterior trim (stain/varnish/paint) and if your walls aren't the same thickness as the door jamb which will be 4 9/16".
Don't forget caulk, shims, insulation, flashing above and below the door, etc. You need help moving around one of those double doors(invite your boyfriend over and make it a date). Have a tarp ready to nail up in case you don't do it in one day. Use a reciprocating saw and a circular saw to cut out the wall. Don't forget that if you need to install a new header, to cut the exterior siding to fit the door opening and the interior wall will have to be cut wider for studs to support the header which will be wider.(Then you may need some drywall or whatever you have to fill in the inside wall.)
First step is to remove your interior window casings to see what you are facing concerning the wall opening.