Hi Philippe,
We have implemented this feature already as DockTabAlignment property. But it was created after the final release of Essential Studio V3.3. If you are interested in recieving a patch build, please create a Direct Trac incident to get the patch version and also mention this forum link.
I have attached the screen shot for your reference. Please refer to it and let me know whether is this the feature you are looking for.
Thanks for your interest in Syncfusion Products.
Regards,
Vijay
Screen Shot