Thanks for your question. Sorry for the slight delay in responding.
There will be a provision in the Lease that prohibits share possession of the premises with others, so provided you do not breach this then there will be no breach of the lease which the landlord could enforce in this regard. You will have to have the lease checked for any other terms specific to that lease and that tenant to ensure that there are no other prohibitions but, generally, I would not expect any.
On that basis you could draw up a partnership agreement confirming what your proposals are but it is important to note that you will not acquire nor shall be able to acquire any interest in the lease in the future. It will remain separate to your agreement and if he defaults on the partnership agreement then although you would be able to sue for the contractual breach you would not be able to compel him to assign (ie. sell) the unexpired residue of the lease to a new tenant - you will simply be able to sue for damages.
Similaly if he tries to assign the lease then you would not be able to restrict the landlord from agreeing to give his consent to this, though again if you have a term in the partnership agreement stating that the tenant should not sell unless you are in agreement then you would be able to sue for contractual breach.
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