Dividing Cuba to help ward off full American takeover?

OK, so let's suppose that, with a post-1780s POD, somehow the UK warns Spain about potential American designs on Cuba, and thus agree to split Cuba in half as a way to help stall any future American designs, whether it be direct annexation or hijacking an independence movement or anything. How would that change Cuban history and politics? Moreover, where would the dividing line be between West Cuba and East Cuba?
 
OK, so let's suppose that, with a post-1780s POD, somehow the UK warns Spain about potential American designs on Cuba, and thus agree to split Cuba in half as a way to help stall any future American designs, whether it be direct annexation or hijacking an independence movement or anything. How would that change Cuban history and politics? Moreover, where would the dividing line be between West Cuba and East Cuba?

I think post-Monroe Doctrine this would be very difficult, and prior to that I doubt the Spanish would be very keen on giving territory to the British as we were fighting them during the American Revolution and some points during the Napoleonic Wars. Maybe the British take the whole island as part of a peace treaty at some point?
 
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