# Makefile.a

.DEFAULT:
	$(MAKE) -f Makefile.b $(MAKECMDGOALS)

# Makefile.b

test1:
	$(MAKE) -f Makefile.a foo bar

test2:
	$(MAKE) -f Makefile.b foo bar

foo:
	@echo foo

bar:
	@echo bar

# make -f Makefile.a test1 gives foo and bar twice
# make -f Makefile.a test2 gives foo and bar once