Discount/cheap airlines in Europe (Ryanair, Wizzair, EasyJet(?)) are very strict about "1 item of hand luggage total" rule. Even if you want to carry 2 very small items instead of one maximum size it's not allowed. I've seen multiple "scandals" with airport personnel enforcing this rule and passengers trying to reason with them based on common logic. Passengers never win and in several cases it ended up with passengers being escorted from the airport. In best case scenarios it ends with 20-40 Euro luggage fee and in-place repacking.
The bottom line is - check the rules of the airline you are flying with and obey them instead of relying on common sense.
After seeing these incidents I try to travel with either only one bag, or, at least, in a configuration that can be assembled into one bag.