I wasn't able to find a great way to manage this. I did it by manually managed the secrets, rotating them as needed to switch back and forth. You could create a script todo it for you - I was migrating workspace accounts so I only needed todo it a few times.
On Mac:
1. Install and initialize Project 1
2. Backup secrets for Project 1
cp /Users/username/bin/gyb/client_secrets.json /Users/username/bin/gyb/client_secrets.json.project1
cp /Users/username/bin/gyb/oauth2service.json /Users/username/bin/gyb/oauth2service.json.project1
3. Remove secrets so you can initialize project 2
rm /Users/username/bin/gyb/client_secrets.json
rm /Users/username/bin/gyb/oauth2service.json
4. Setup Project 2 - I just ran the installer again, followed the instructions to setup the other workspace account
bash <(curl -s -S -L
https://gam-shortn.appspot.com/gam-install)
5. Backup secrets for project2
cp /Users/username/bin/gyb/client_secrets.json /Users/username/bin/gyb/client_secrets.json.project1
cp /Users/username/bin/gyb/oauth2service.json /Users/username/bin/gyb/oauth2service.json.project1
Now to switch between them:
1. Use Project 1
cp /Users/username/bin/gyb/client_secrets.json.project1 /Users/username/bin/gyb/client_secrets.json
cp /Users/username/bin/gyb/oauth2service.json.project1 /Users/username/bin/gyb/oauth2service.json
2. Use Project 2
cp /Users/username/bin/gyb/client_secrets.json.project2 /Users/username/bin/gyb/client_secrets.json
cp /Users/username/bin/gyb/oauth2service.json.project2 /Users/username/bin/gyb/oauth2service.json