Yes, there can be more than one Client. Regulation 8 allows for the election in writing of one client if all parties agree: if not, there can be more than one client. However if this is the case, they will all attract the Client's duties.

There can be, but in the circumstances described I don't foresee either client wanting that to be the case. As I understand it they will each effectively become joint and severally liable for all the work (though I'm not aware of this actually being tested in law - does anyone know otherwise?) That is, the store client will potentially be liable for any CDM failure in the aspects of work which might otherwise be clearly the responsibility of the other client, and vice versa. I don't believe CDM requires a lead client, though it strongly encourages it (see ACOP para 42).

The work is always done as one project and the store project manager looks after both jobs backed up by the other PM. Only one fee I'm afraid.