As this is allowed, is it also allowed to refer the one account by the other one to get referral comissions? (Of course using Free Rolls only on one account)
No, that would be abusing the referral system. There is no rule against having 2 accounts but the referral system is meant to reward you for introducing new people to the site, referring yourself obviously doesn't fit into that. It is really just a matter of common sense as what is and isn't abuse.
so... how about those wagering and referral contest winners?
some of them are suspiciously looked like they referred their own account to get both prizes
the connected accounts in question, both are having the same total wager amounts
The fact that an account only has one referral that wagers doesn't make it suspicious. These accounts certainly didn't do it to abuse the wagering contest as they are much older than the competition.