I use two, so I can also assign them a custom D Book with the majority of the 6-8 man blitzes removed (too easy to exploit)...
You can do it with one though...
You have to:
Pause
Change sides
Go past the "changes will take place after the next play"
Back into the game / It will still show YOUR playcall screen, so:
You "Sim" but hit "exit" before you actually sim and BOOM:
You're in the other teams playcall screen where you can make adjustments and the repat the process to back out and end up where you started...
Kind of hard to explain, but once you get the hang, takes seconds...
Or: Use Two!