Hi ,
hope i really understood your question,
In standard , if the Relase strategy is configured correctly then the release strategy will trigger for PO with same plant across all line item.
The release strategy cant be determind for multiple Plants in single PO in standard config. you can try using the blank value for aggregates.
refer following SAP notes for more informaiton.
47089 - Release procedure for purching docs - Aggregation
493900 - FAQ: Release Strategy
Thanks
Kunal