I think you could use:
class Module12(Module1, Module2):
	def __init__(self):
		super(Module12, self).__init__()
		if hasattr(self, 'l1_loss'):
			print('l1_loss at position 1')

		if hasattr(self, 'l1_loss'):
			print('l1_loss at position 2')

m = Module12()

which will print both statements.