Currently with AR, the issue hierarchy doesn't support one child to roll-up to multiple parent. It is a one for one relationship. Just like a STORY can only be associated to one EPIC and not multiple EPICs.
One possible workaround (however it will not be displayed in AR plan at this time) is for one to use LINKED issue to associate the EPIC with multiple Initiatives.
Hope this helps.
Best, Joseph Chung Yin
Jira/JSM Functional Lead, Global Infrastructure Applications Team