Yes, the field names on the two post types would have to be the same to get the posts intermingled.
In order to use 2 different field names the fields must have some relationship to each other.
The only other options is would be to do two different queries, add the value of the acf field as a property of each post object, merge the results of both and then manually sort them in one of the php array sorting functions, likely with usort().
