Lets say you created a field named ‘my_option’ in two different field groups. Each field is given a unique field key by ACF. This field key is used by ACF to determine what to do with the content that’s stored in the field. If the field in both groups have identical settings then it won’t be a problem. If they don’t then there will because when ACF saves a field it associated value with the field key of the last save. Since you plan to use the fields in the same way, just edit them in two places, then it won’t be a problem.
Another thing when considering acf_form() is that you can specify the fields that you want to use rather than the field group, so there isn’t really a need to create another field group, just pick and choose the fields from the groups on options pages that you want them to edit. … completely forgot about this. I’m aware of all of the things it can do, I just haven’t had the need to use acf_form().
There are several ways to use field groups on multiple sites in multisite, but like I said, there isn’t a way to create a field in the admin of one site and have it available on all sites. You either need to define the groups using PHP in a file that’s common to all sites or use the local_json feature to set up a load point and this setting needs to be common to all sites.
