Multi boot, BUT...
 

I'd like to install on my parents computer two different distributions.
Is it possible to enter an username after turning the computer on and based on that username one specific distribution will be booted?

I don't know about that, but you could always turn off the boot time out for Grub and just change the menu so one reads "Mom -- Gentoo" and the other read "Dad -- Slackware".

Yeah, this is my plan b, but i'm searching for a more "elegant" way...

More elegant than hitting an arrow key and "enter"?

I don't think boot loaders typically prompt for a login (other than a security feature, I mean). You could use the same distro and just configure a different desktop for each of them. Then when they login at GDM (or KDM) it will remember the desktop used by each person. But that's only really helpful if one uses Gnome and one uses KDE (though you could customize two different KDE desktop setups).

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Well, i think more elegant is NOT "hitting an arrow key and "enter"" (and after that a username and password), but only entering username and password.
If it's not a boot loaders thingy, is it possible to do that with a Bios thingy or idontknowwhatelse?

That's the KISS idea...

Nothing that I can think of right now.

However, you could do what they told you above, and set up both distros to log-in automatically without user intervention.

That way, you would select with arrow, hit enter, and wait until your desktop is up. How to set up automatic login will vary from distro to distro slightly.