You need to go talk to the landlord. Explain that you can't afford to cover R's 1/2 of the rent, but that you have a new tenant lined up to take over as soon as L gets R out for you. Ask L to evict just R & not you. L should go for this deal: L will have to evict someone anyway, & if L lets you stay with the new person, L doesn't have to worry about advertising & showing the place.
Also, since you have a person lined up, if L doesn't follow your plan, you have a very good argument that L did not mitigate damages & could have saved advertising costs & recouped rent faster if L had done what you wanted.
Write L explaining all this & pointing out that you will make the failure to mitigate argument in court. Send the letter certified, return receipt. Talk to L face-to-face also. The letter is just so you can prove L knew of your plan & unreasonably rejected it.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
