Is it possible to default a shared value field but allow overwrite

I have two fields multiple choice fields on different sections of a single form. The second field uses shared values from the first field.

I need to be able to present, in the second field, the content of the first field (as the default entry) but i also need to allow the user to overwrite that default entry by selecting one of the other options where required.

I can get all of this working quite simply, with the exception of being able to initially default the second field. I thought i could use form logic (i.e. if the first field has content and the second field does not then change the second field to the content of the first field) but i am not allowed to set the value of a field that uses shared values.