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How about keeping the projects separate for ease of working as you mentioned, but then also create one main project. You can just set up rules that automatically multi-home tasks, more info here: Using Rules in Asana | Product Guide • Asana Product Guide, Multi-Home tasks automatically in two projects - #2 by Andrea_Mayer
Though portfolio is great as well as it offers a lot of features: How to Get Started With Asana Portfolios | Product Guide • Asana Product Guide
And then you can also make use of the Workload feature see View Your Team's Workload in Asana | Product Guide • Asana Product Guide
I hope that helps