7 ohm coils are not going to produce much vapour even at 5v. And I'm not sure anyone sells anything like that as it would suit much higher voltages. You can think of a dual coil cartomizer as the same as running two atty's. Electrically it's the same.
You can use the 1.5 ohm dual coil carto's at 5v. You would be running two 3 ohm coils at 5v which is fine. However, this is going to draw a lot of current (3.3A) so you will need high drain batteries to discharge that much current. Some mods have a regulated maximum current output as well (e.g. my 5v box mod has a current limit of 3A with a safety cut off beyond that) so thats something to bear in mind. Of course a single coil 3 ohm carto will not have these issues.