There are several options. You refer to two forms; are these forms for different entities, and if so, how are the entities related ?
If the relationship is a One-Many (Parent-Child), and users only update data on the parent entity of the 2, then you can use a calculated field on the child so that it takes the value from the parent.
Workflow or javascript may be other options, though it may get complicated if you want to be able to edit the field on either form/entity
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