Yes, you can merge multiple accounts.
There are two ways to merge accounts, both with different results.
First there is the possibility to merge two accounts that belong to the same company. When you merge these accounts, the conversations from both accounts will all be placed in the resulting merged account. This includes all data linked to those conversations, such as messages, attachments, etcetera. Both accounts have literally become one. A little warning: this cannot be undone.
Then there is the option of merging two accounts that belong to different companies. When you merge these accounts, all data will stay in the separate accounts. The only thing that really changes is that at the top of the screen (in the web interface at the top left side) you can click on the name of the company. This will show you a drop-down menu, with all of your companies. Clicking on one of those company names makes you go to the account that belongs to that company. This way of merging accounts can be undone.
To merge two accounts, go to ‘Profile’ in the web version. Then choose ‘Advanced’ and click ‘Merge’. You will be prompted to log in with the account that you want to merge with the account you are currently in. When you have done that, the accounts are merged.
To separate your accounts again, go back to ‘Advanced’, via the ‘Profile settings’. This time click ‘Separate’. The account you are currently logged in with will be separated from your other account(s).