How to have the same Product Backlog for two projects

There are three ways to achieve this:

1. With separate projects. An extra project is then set up for the backlog. This project can also provide any cross-project reporting that is 
required. As a story is assigned to a project it is then copied/moved to that project using the Mingle copy facility. The downside of this is that 
the backlog is held separately to the work in progress and team members  must look in 2 places. Stories can only be moved individually.

2. The term “project” in Mingle is a very abstract term. A number of  clients use a single Mingle “Project” across multiple development efforts. 
Effectively they add a “project” or sometimes “team” card to the top level  of their trees. The downside of this approach is that a number of identical 
favorites and tabs need to be set up – identical except for a filter on the project for each of the projects.

3. Adding a property of “project” to the relevant card level, epic,  feature or story. This allows the project to be changed when it is 
assigned, unassigned is the “not set” value (or a status defined as “backlog”). The downsides are very similar to the previous suggestion.

Which suggestion is the best for you will depend, at least partially, on your reporting requirements. All have some issues because MQL is limited in 
how it can hold what are, in effect, user variables (as distinct to project variables), this is addressed by different teams having different 
prefixes to their ids or by adding users as cards with the project they are currently assigned to.