Here are a couple of options:
use a connected app to authenticate + APEX in org A to authenticate and use the flow in org B. The downside for this option is (1) flow UI screens cannot be re-used and (2) this option is best for auto-launched flows only
use Canvas
- https://developer.salesforce.com/docs/atlas.en-us.platform_connect.meta/platform_connect/canvas_framework_intro.htm
- https://help.salesforce.com/articleView?id=flow_distribute_internal_vf.htm&type=5
- https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/components_using_flow_cmp.htm
