I just took a short nap and so much has happened!
"I spent the whole night just on the threshold ...... Ridiculous." So my first question is: Is this a separate threshold or an integrated part of the total jamb? I'll assume it is separate, installed and the jamb will sit on top of it. Make sure it is level or you will have to adjust the length of the jamb legs to ensure that the header (top piece) is absolutely level. You can also scribe the leg bottoms to make an exact fit if the threshold is off level or twisted.
The reason you normally shim the hinge side first is to ensure that the imaginary vertical line through all of the hinge pins is plumb so that the door hangs properly (doesn't swing open or closed on it's own). Then shim the latch side to fit the door. But since your rough opening is absolutely plumb in both directions (sideways and in/out) and square, we don't have to worry about that. Right?
Pick the most plumb side and make it perfect! Attach both jams to it first and shim the other side to make it square and measuring consistently top to bottom. Don't cut the shims until the the doors are hung and aligned...just in case a little tweaking is required. (You know how to test your level for accuracy?...check it one way, then turn it end to end and check again using the same side of the level on the surface. Even if the surface isn't exactly plumb or level it should read exactly the same either way. If it doesn't go to the store and test your new one before you buy it.
A very large percentage of levels are not true to begin with!)
In the diagram you haven't indicated if there are any walls adjacent to the doors on either side (it's usually nice to have a door open against a wall rather than into the room). I'll assume that you have that worked out. With a 12" jamb, the knobs wouldn't hit each other, if they were hinged on the same side. My doors will be hinged on the same side simply because that is how they open onto walls inside and out. I am not aware of any code restrictions that would prevent you from having the doors oriented this way. Our exterior doors are often in much thinner walls and therefore have the main door and screen/storm doors oriented this way because the handles would hit.
Tim has a good thought about the decoupling and covering the joint with a "burlap". (how do you think I feel when you guys use acoustical jargon and related acronyms! ...that you often can't find with Google!
) Your in my playground now! Another reason to make the jamb in two pieces is because it is 12" wide! The wood will most likely move, probably shrinking, but if the wood is very dry it could even grow if you have a humid climate or if varies seasonally. Do you even have seasons? I may have to come out there to investigate!
Anyways, if it fastened solidly to hold two heavy doors it would crack if it couldn't move. It wouldn't hurt to have a 1/4" or 3/8" gap. You could help to minimize this movement by sealing all sides and ends of the jamb before installation (only because this is such a critical doorway.) Where is this MDF? Is it coupling? Is there any real wood behind it or could you add a piece? If not, could you replace it with Plywood, which would hold a screw or nail better? Detail pictures of the rough opening?
You could hide a screw behind each hinge that could angle into the rough framing if the DW is too thick to really make the door solid. Or even behind the surface mounted door seals. I'll often put screw through the jamb, at the shim locations, under where the door stops go when I'm hanging heavy doors.
So is this the seal you are using? http://www.zerointernational.com/catalogpage.aspx?pageID=53
It is surface mounted!
If you had a rebated/rabbeted jam similar to an exterior door (over here) there is usually a 1/2" of integral "stop" that is big enough to take a similar 0 gasket that fits into a narrow saw kerf that would be hidden when the door is closed. See bottom right... http://www.ultrafab.com/products/bulb_seals/bulb_seals.htm
However "yours" is adjustable if the door develops a bit of a twist which is a bonus! You could rap it with mahogany veneer after everything is adjusted...and settled.
So I've been playing with my painter in the driveway trying to get 115 4x4 turned balusters stained. I had the 1st coat brushed on waiting for two coats of sprayed-on. Wednesday, a beautiful day, was wasted setting up and buying replacement spray guns (first was DOA and TWO new defective ones), Thursday was productive but getting humid. Today was raining all day. So we had to sit drinking coffee until we broke for lunch. Then more rain watching til 4 when we broke out the Scotch. It's brutal out there!
I wasn't going to say anything about these doors being your first...just take your time and check your hinge layout before you use the knife. You are matching the jambs to the doors. If you screw up a jamb, it is easily replaced! You've been exhibiting great craftsmanship so far!