Completely synchronise multiple Macs I actually have a Macbook Pro with Snow Leopard (I plan on upgrading to Lion).

I may have to buy an Mac mini in order to have a full dual screen (external identical screens) layout (see my previous question).

So I will work on a Mac Mini and a Macbook Pro, most of the time on the same LAN.

How to get everything completely synchronised, as fast as possible? (I don't know if iCloud can)

I am talking about every file/data on my computer:


applications
application preferences
mails
calendar
iPhoto/aperture libraries
lots of small files (web development...)
files out of my account (apache server files, php/libraries)
...


I don't want to manage 2 computers and do everything twice (installing something, configuring the system, etc...).
Try using Mac OS X Server installed on the Mac Mini. Technically you should be able to install configure and mange users, applications etc all on the Mac Mini. Then you should be able to login via the MacBook Pro as a user on the server and pull down a configuration of Apps etc that you set up on server for your user accounts. See Considering OS X Server for home users.

This is just one example and it may be irrelevant to some people, so to understand the scope of what OS X Server has to offer it is best to look at all of its services lumped into four general categories: Account Management, File Sharing, Network Administration, and Web Services.
User account management
...
Beyond the directory itself are the domain management features of OS X, with which you can bind client Mac computers (such as your desktop iMac) to the server so the usernames and passwords in the server can be used to log in to the client Mac system. With this setup, you do not need to make new accounts on a system every time you want to allow someone to use a computer. Instead, you can go to the server and enable an account for a workstation, and that user can go log in to that system and use it. Additionally, if you purchase new systems all you need to do is bind them to the server and users can then log in to them with the same restrictions that they have on your other Macs (such as access only to certain applications, and only specific settings being available to them).

Also check out On Mobile Accounts and Syncing  where there is more research to be done at on how well OS X server does mobile accounts. It looks like Apples Offial name is Portable Home Directories as referenced from OS X Lion server spec under Client Management, this should allowing syncing to occur for your files in your home directory.