Resource allocation across different projects

There are people who will be doing work on two different projects. For convenience, let’s say A’s time will be spent on Project 1 is 70% and Project 2 is 30%. I want to see the impact of allocating A to Project 2 in Project 1. e.g. will it delay Project 1 completion date? How do I do this in Primavera? I notice that I can’t use the calendar as the resource calendar will be used across different project. Do I change the Remaining Units/Time on each activity?