cjgrass,
The 'brother in law' qualification could be performed provided the requisite number of coupons are destructively tested and pass. Your statment "one would weld one side and one would weld the other side of the test coupon" was meant to believe that each would weld one half of the circumference, not one half of the thickness, the latter not permitted.
While testing at 6G would qualify for butt welds and lap fillet welds in all positions, each welder would still be restricted by the many other essential variables for single qualification. I've seen this done many times in the past, and eventally the welder will make a weld for which their single qualification was not approproate, resulting in a cutout.
However, in addition to the above test, a branch test performed to API 1104 6.3.1 (each welder will need to make their own weld) will impart multiple qualification which will make them more valueable to you and your organization in the long run.