A disconnected node is treated as black, so a Multiply would multiply black by the second texture, which will always be black. And there would be no reason to disconnect texture 1 after a connection has been made. So I think this is working as expected.But..multiply texture...now it has 2 texture (default: one is not connected and one is rgb)? And it seems not to work properly with preview node. Evenmore if I put an image on texture 1 ...after I can't disconnect it.
Paul