Hello,
There isn't a way to merge two screen together. You will have to create a new screen based on one of the two tables. Then, add link fields from the second table to the first table. Now you have access to fields in both tables for the one screen. Take a look at the Activity screens and table.
For copying a field to another section, this is not possible. You can create a textbox and on click of a button, the value in the textbox can be saved for the field. You run into a logic problem with copying the same field to different sections of a screen. When you do a Save All Sections, how will the system know what value to store for the field if there are two different values inputed for the same field ?
Greg