So it occurred to me I'm not sure how o install our french doors in our basement. We are using the Amvic block with 6.5" of foam and 8" of concrete for a 14.5" wide block in total. If I frame out the door bucks just like the window bucks, I will have foam sticking out on either side of the threshold/sill. Foam that can't be stepped on. My only idea is to use two 2x's in parallel, ripped so that they are wide enough to cover the 14.5" wide block, and have the seam hidden under the door frame once installed. The problem I see with this is cantilevering a pine board 4" or so over the edge of the foam where there will be no support. Is there a better way to do this? Should I do as described above but carve out some foam so that I have more concrete supporting the threshold piece?

There are many good methods to cure this problem. Ripping 2x's would be satisfactory I guess as long as they were both anchored into the concrete. I have also had this same method to me by architects/ GC's, but with a plywood backer ripped to the full width to add support. I do not care much for either of these methods and understand your uneasiness regarding the longevity of this structural design. The method I use most often to cure this type of issue is to rip a center board (in this case a 2 x 14 available now through many lumber suppliers ripped down to11.5" ) and then firmly fasten 2x material to each side and anchor all to concrete. This method also provides a larger fastening medium for interior and exterior needs. Codes in most areas will require a moisture barrier on all surfaces that will come in contact with concrete. This buck system may not be as simple as a mere strip of ply or 2x but is also not not very difficult to achieve and i have yet to receive a complaint on their appearance or structural integrity.

The method I use most often to cure this type of issue is to rip a center board (in this case a 2 x 14 available now through many lumber suppliers ripped down to11.5" ) and then firmly fasten 2x material to each side and anchor all to concrete."

Could you explain that again? I'm not sure what you mean

Sorry about that. Words aren't my gift. Using a table saw I would rip 2 x 14 material down to 11.5". Cut your sides, top and bottom to length. Fasten sides top and bottom of this 11.5 inch board with 3 1/2 inch screws so that you have the rough opening desired. To achieve your desired width you will need to attach 2x 3 or 2 x 4 material at 90 degrees to both the interior and exterior of the buck, keeping the 2 x's flush with your rough opening dimension. This will add 3 inches to your 11.5. The appearance is that of a C channel. To add strength, fasteners can be installed through both the center pieces of your bucking as well as the 2 x 4 on interior and exterior prior to concrete placement. This will avail not only a very study threshold but also an excellent attachment medium for the door frame. If I have failed yet in communicating this process perhaps we can talk via phone. Let me know.

OK I follow now. You end up with a 14.5" piece for the bottom of the door opening. I assume then you must remove some foam as to recess the faces of those 2x4s end up flush with the wall?

Yes, the 2 x 4s are flush with the inside and outside of the foam block. It is easier to cut the full opening and then reattach foam insulation to the buck to avoid a little thermal bridging.

Thanks for your help.