Agile - one backlog with multiple agile teams

[#1]

We want to work with more than one agile team on the same backlog for a complex product/market/solution as described in the Scrum Guide. That is necessary to keep the teams small but have all requirements in a single product backlog.

Therefore we need to be able to assign more than one agile team to an task/EPIC/Story/etc. Today we can assign only one single team to a story which means it isn't visible to the other teams anymore.