This used to worry me a lot when i first went over to wfp.
If you have a tap on your belt or a aquadpter just switch off, wipe the sills a couple of times, then go on to the next upstairs window.
Usually by now the drips on the first window have eased off, so proceed to clean the window underneath.
I know its a bit scarey but i honestly havnt had any problems.
If you have rinsed well upstairs and the sills below are clean, then any lil drops crashing down will have a low to zero tds