It is possible to have the app competible with iOS 12 and iOS 13 at the same time published on the Apple App Store?
keep both old and new app version in Apple store
No. But why would you want to do this? The app itself is capable of detecting the iOS version
If you publish an update that specifies a minimum required iOS version of 13, then users who have previously purchased your app with devices that cannot update beyond iOS 12 will be able to continue to download the last-compatible version.
Because I want to update the minimum deployment from 12 to 13 and the app has some users with iOS 12 version.