My husband & I work for a window company in the midwest. We are installers. If you go with a reliable window company, (do your homework check each one out) they should be able to replace with twin double hungs and can make the opening bigger. Make sure with the window company how long their installers have been installing windows. Depending on how big the opening is: The original header should still be there from before. If not a new one needs to be put in. Depending on the opening size twin dble hungs do look nice, you do not want to go over 36" in width for each window that makes for a large window and can be awkward to clean if you get the tilt in windows to clean. This would mean an opening at least 76-80" in width. The height makes a difference also. Generally a standard dble hung window is 28-36" in width & 45-57" in height. Picture windows do very in size, This will not be an 2-3hour deal, if you want to make it bigger that means cutting a hole and filling in and putting up new trim.
If you dont want to make it bigger, less work, and can be done in a short time. Good luck and do hour homework

Windowlady - Thank you so much for all the info! This is very helpful. I will definitely use it as a starting point for my research.