Hi Thierry,
No, this isn't supported at the moment. Allowing multiple columns within rich text would be the wrong approach - that would mean you'd have page layout information embedded within article content, and the whole idea of a content management system is to keep those separate.
We've considered allowing more customisation of the panels system, to allow displaying fields horizontally in the editor - for example, you could construct a MultiFieldPanel containing two rich text fields, and tell it to display those horizontally rather than vertically. However, this feature isn't a priority for us - it would require a lot of design work to make it look acceptable, and again, it doesn't really fit Wagtail's philosophy of keeping content and presentation separate. Ideally, editors should only be thinking about the pure information content of the page - not how it will be displayed.
Cheers,
- Matt