Answers to your questions:
1) You can write values in 2 tags by using scripts. You can do it for example by using tag dependent ST script. When tag's become some value script is called and write value to other tag or you write in 2 tags if you want.
2) You can also do it by using scripts. You can do it for example by using screen dependent ST script. You can call this script when the screen is closed by using OnClose event and bind this script to this script. In this script you can write value to the tag.
3) The same way you can do for open screen, buy use OnOpen event for calling script.