Set variable on a related Form B with data from Form A before Form A has been submitted/persisted. Possible?

Set variable on a related Form B with data from Form A before Form A has been submitted/persisted. Possible?

Form A has a NameA field with data "John Doe"   
A related Form B has a field NameB that is to accessed half way through filling out Form A.
How do I set the value for Name B as "John Doe" before Form A is submitted/persisted?