I have two cherry tree saplings here in Bogotá Colombia. It's sunny and not raining much, with 20 °C (80 °F) During the day, dropping to around 8/9 °C at night (47 °F). I'm watering around a cup of water or a bit more every two or three days depending on how sunny it is. The saplings are in large separate pots in a balcony, behind other pots so as to not suffer from wind. I want to ask if it's too late and if not what steps could be taken, and what seems to be the issue.
They're dying of thirst - I don't know what you mean by a 'cup' but if you mean like a teacupful, that's nowhere near enough, it won't even be reaching the roots. If its 20degC and sunny in the day, they probably need a litre or two every other day if not daily when it's hotter, assuming the pots have drainage holes for any excess to drain away. Give them a thorough soaking asap, then water properly ongoing.