Do you have any experience as to whether you can be logged into Marketo with the same account on several devices in parallel and edit assets etc. without this leading to problems (changes are not adopted or the like)?
I have logged in to Marketo in multiple browsers and it was not a problem. In general you don't want multiple sessions (or multiple users for that matter) editing the same email at the same time. Have you tested logging in with different browsers/devices at the same time?
That said, I do not recommend sharing logins between multiple people, that makes it really hard to audit and control what's going on in your system.