Roll up project plans into one master plan?

Hello everyone and thanks in advance for any input!

Is there a way to roll up two separate project plans into one "master project plan"?

For example, if one were implementing a CRM (which has its own project plan) while at the same time implementing a new HR system (which also has its own project plan) - AND - these two projects are part of a single Initiative, is there a way roll these two projects into one "master project plan"?

I welcome your ideas!

Thank you,