I am implementing GA4 in one app (iOS and Android). I want to send data from this app to two different GA4 properties - one with historical data and one brand new. I don't want to take BigQuery or any programmatic road. can I add google-service.json config files from 2 GA4 properties in a single app?
Well, the short answer is no. Because the library (firebase SDK) only allows for there to be one init and one, effectively, "tracker" object. What you're saying is not even a popular usecase for two GA4 properties. The real life usecase is sending events to a Web+App and an App properties.
Keep in mind, GA4 is an unfinished product, as well as its libraries. Many bugs, absent features and holes in functionality. We generally recommend to not use GA4 for a year or two or only implement very basic tracking in parallel with whatever you use now just to have historical data when it's time to switch.
What you could do if you really need to send the events to two GA4 properties, is to use the GA4's measurement protocol, for which, as you expect, there's few good third party libraries and no proper documentation. I've seen people using it here on SO, however.