In addition to what Ed mentioned, I'd like to offer some more thoughts. I have double-entry, solid oak, full-oval doors. (You didn't mention what types of doors you have so that's what prompted me to assume you might have doors similiar to mine.) One is active and the other is passive.
(For the benefit of others ...) A door mullion is the metal device used where double doors are installed and is attached to the door frame at the ceiling and to a steel plate in the floor. The doors close by latching to this device. When properly installed it is secured at the top and bottom. Moving right along ...
Another assumption I'm gonna make is that you're installing these doors in your home. Have you considered using flush bolts? With flush bolts installed in the dummy door, you can install a mortised lockset into the active door where it 'latches' into the dummy door. Hence, the dummy door acts like a mullion AND with flush bolts in the dummy door, there aren't any unsightly, visible locks/latches holding it closed. With the aforementioned type of lockset, there's no deadbolt to be seen!
Baldwin has all this hardware. I used a mortised lockset with emergency egress. It was the Lexington. You can find them very inexpensively on the WEB. I saved over $300 by buying it, and the dummy set, on the WEB. I also have storm doors (along with a 6' full-front porch overhang.) We only get some winter sun and the 'space' between the doors does heat up. In fact, the mfgr of the doors only warrantied the doors for 1 year instead of 3. (We face due south, thank God for the porch!) If you have metal doors, using flush bolts is worth looking into. If you need more info, e-mail me directly.
My best to ya and hope this helps.
PS: Oh, I forgot to explicitly mention that by using flush bolts and the mortised lock, there's no obstruction should you want to open both doors (say, to move in some furniture.)