Need extra Show/Hide function so that a field is not completely locked out.

Hi, I can use the Actions>Mouse Up>show/hide function to force users to enter data into one of two fields, but once they enter data in the first field, they cannot change their mind and then enter data into the second field, the second field is inaccessible even if they erase the data in the first field.  Is there a built-in function or javascript that would allow users to enter info in one field, then if they change their mind, click on the other field to enter data and have the first field’s data automatically deleted?

Thanks!