Huh? Doesn't this work:

    all: copied o a
    source:
    	echo "message" > source
    foo.o foo.a: source
    	(echo -n ".o: "; cat source) > foo.o
    	(echo -n ".a: "; cat source) > foo.a
    o: foo.o
    	cat foo.o > o
    a: foo.a
    	cat foo.a > a
    copied: foo.o foo.a
    	cat foo.o foo.a > copied

The third rule simulates a compiler producing two outputs. Now if foo.o changes, both "copied" and "o" will be updated, and if foo.a changes, both "copied" and "a" will be updated. (And if either foo.o or foo.a are deleted, the compiler will be rerun, as will everything depending on foo.a or foo.o.)

This is gnu make 4.0.