Lamb & Wolf Separate champs
I believe The lamb and wolf is actually the first duo champion designed to directly affect each other and actually fill two different roles as detailed below.
The Wolf He is the jungler and always roams and hunts for kills whenever possible.
The Lamb She could be either a mid or top laner but i think mid lane is more likely just based on her voice :p
why not bot lane? Well that is because riot has told us they are not an assassin or a support, though i suppose the lamb could be an adc but that doesn't really fit with the theme.
How they work together While both live they each have their own skills that is directly related to the overall theme of wolf or lamb; when one dies and the other lives is when these two get interesting as i suspect that if the lamb dies the wolfs abilities change in effect and vise versa, what these changes will be only riot knows though i suspect it is based off of garens villain system but includes assists as well.
Dude Champ select wouldn't allow for this! Ah but it could easily as both of them could stand as their own unique champion thus could be banned independently, this is where i think the masks on death come into play so that if either one is banned they can still get their unique transformation powers by assisting or getting kills thus earning mask transformation stacks, in short when only one is on the team this mask system fills in for the other champion not being picked.
What if each side picks one? This could be handled two ways, the first being if you pick the lamb or the wolf it automatically makes it so ONLY your team can pick the other champion, alternatively both could rely on the mask system when on opposite teams though i think the first option is more likely and fits more theme wise.
Disclaimer I could be 100% wrong on this, it is nothing more then my opinion on the subject and is only meant as a discussion point based on the limited information we currently have.