There are two ways to approach this. One being a simple way and make the field Global binding. The other way is writing JavaScript code, e.g if you had this type of hierarchy:
Page1
Subform1
DropDownList1
Page2
Subform2
TextField2
On the Exit event, with JavaScript language selected, your code would look something like this:
xfa.resolveNode("form1.Page2.Subform2 ["+this.parent.index+"]").TextField2.rawValue = xfa.resolveNode("form1.Page1. Subform1["+this.parent.index+"]").DropDownList1.rawValue;
Make your TextField2 on the second page as ReadOnly so that changes cannot be made to it.
I hope this helps.