In Activity Stream, how is the 'active group' determined when a node has multiple organic groups assigned to it?
I believe the active group is just the group context which is present when content is created (i.e. if you're 'in' group B and post a node into groups A and B, it'll only go into the stream of group B). So it's not great for cross-posting.

You shouldn't use the patch in that issue because it was never finished, it's out of date, and has bugs and bad code that would most likely break your site.

If you plan to keep using Commons 6.x, best to post a feature request in the Activity log queue requesting default rules that would work to create multiple log messages when content is cross-posted across groups. Otherwise, wait for Commons 7.x where it's not planned to use activity log.