Yes it is possible. You can buy both license in one account,but one user cannot assign both license because it will override the permission otherwise manually deselect some option under their plan
Just for your knowledge
E1 Plan below
Mobile Device Management for Office 365
Yammer Enterprise
Skype for Business Online
SharePoint Online
Exchange Online
E3 Plan has
Mobile Device Management for Office 365
Yammer Enterprise
Azure Rights Management
Office 365 ProPlus
Skype for Business Online >
Office Online
SharePoint Online
Exchange Online
Project Online
Project Online
Office Online
SharePoint Online
If you activate project online and E3, it will not work because project online has office online and share point online , so you have to deactivate this product any one of the service
Regards,
Joby
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Edited by
Joby M Chacko
17 hours 15 minutes ago