I can think of 2 solution.
The new date picker (actually there are 2 new date pickers, one in canvas apps, and one in Teams app and components. The Teams/Component has this feature, the other has not. You have to go to setting in your app to turn them on. Search modern controller. To add it to a Canvas app, start a new component, add it to the "app" and copy it and paste it into you canvas app.
The new date picker is way better than the old, but keep in mind its not datePicer.SelectedDate, but DatePicker.Value and some other minor changes.
In advanced you can set minimum date or something like that, locking the ability to select date before that time. This could be from input from another date picker as you need.
Old fashion solution
The other solution is to change color to date text to red if it is < StartDate.SelectedDate, add a warning text (if you want) and disable the "save button" if endDate.SelectedDate < StartDate.SelectedDate
