Zervant is a cloud-based software, therefore, when using two or more accounts at the same time on the same browser, it is highly likely that problems occur with data integrity while using Zervant. This is also related to the browser cache and account data that is stored during the duration of your login session.
Things that may happen are invoices or customers being saved in the wrong account or wrong data displayed in different views. These issues are not Zervant-specific and the same type of behaviour can occur when using any web app while you are logged in with two or more accounts at the same time.
If more accounts have to be used at the same time on the same computer, the best way is to use different browsers for different accounts (e.g. Google Chrome for account 1 and Mozilla Firefox for account 2).