Unless the dwelling is a legal two-family house(duplex, etc), the life estate extends to the entire property. In a legal duplex you could grant a life estate to the one property. The key here is that there would be a deed for each property. A life estate is a deeded interest(it is essentially a right of property use until death) and cannot be "taken back" except with their consent or a breach of certain terms in the deed(usually life estates cannot be encumbered/mortgaged by the person holding them and often there are provisions for paying utilities, maintenance, ownership costs, etc). They would need to give up their right(possibly by quit-claim deed? or other conveyance) prior to you being able to sell it. They may negotiate with you terms for doing so, such as finding them a new place, etc. You all just have to agree. Often, life estates are granted with clauses that state the person(s) actually use/inhabit the property(often they do not have to be occupied full time such as with older snowbirds) - abandonment can terminate the estate.
Talk to a lawyer about getting a good agreement written that protects the interests of both parties.

