Can i take a half dose of dayquil then 2 hours later take half a dose of nyquil? like when its 8 pm and want to stay up till 10 pm for example. since a full dose is supposed to be every four hours?

Probably not ideal. There isn’t anything in DayQuil that should keep you awake, but there are ingredients in NyQuil that may make you drowsy. Please check this website for info: https://sleepauthorities.com/does-dayquil-keep-you-awake/ The half dose of DayQuil may or may not be enough to quell your symptoms, and will not be fully metabolized when you take that later dose of NyQuil.