right on, kk,
the only way to attach the two blocks in that way would be to sew a strip on the backing and flip to the front, sew them together, and then turn under the edges and top stitch.
it defeats the purpose of SAYG or F&D, and makes it harder to do because you're adding the extra step of adding the strip, which otherwise would be built in. clear as mud?. only if you have blocks already made would this be a good idea.
oh. wait. did kid124 mean an undone block backing only? if yes, then i haven't understoond it well. sorry.