If AppServices are not an option, you can expose the Microflow as a SOAP webservice in App B, and consume this webservice from App A.
Try app services, https://world.mendix.com/display/refguide5/Published+App+Services
For Mendix 4, use web services:
https://world.mendix.com/display/refguide4/Published+Web+Services
https://world.mendix.com/display/refguide4/Imported+Web+Services
Hi Michael... above App in not available in 4.7.1....