Far fetched but I actually think the J-36 and J-50 are both aiming for the same role and in active competition. J-50 being the more technologically riskier design. The size difference of the J-36 is to accommodate a 3rd engine, curved inlet duct and larger fuel tanks to make up for extra engine whereas the J-50 bet on more novel technologies to reach the same energy generation and/or speed and/or range being in a smaller package.
Perhaps the lockheed design is also a technologically more conservative but larger aircraft compared to boeing just from the artwork we've seen. Unlike boeing, lockheed artwork was pretty consistent throughout, that of a f-111 sized aircraft.
Yes I'm aware that a few very reliable chinese sources have claimed that they are built for 2 different programs, but these sources never made known that it was whether from their own observation or inside knowledge.