We see this question come up from time to time. The quick answer is yes, you can add your Memberstack head script to multiple sites! Doing this allows your members to login to both websites but there are some drawbacks.
You can’t test both sites at once in the Memberstack app.
You’ll need to match the file structure/names on both sites otherwise redirects will not work properly. The beginning of the url in the Memberstack app is relative meaning that we only use the last part of the url for the redirects.
If a member logs into one site and clicks through to the other site they will appear logged out because each site is unaware of the other (member cookies are stored domain specific). If they log into the new site they will essentially have two “active sessions” going.