Ok, I managed to carry 2 different passenger on my bike. Here is what it looks like now:
At first, when they see the seat, they are not really convinced about it, but once they get on the seat they get quite happy about it.
I managed to bring a girl to a restaurant (6km distance) and bring her back home with the bike
But obviously, a bicycle is not something to use to pick up girl because they are at first disappointed!
Now, in term of issue, the handling is not very good. The seat probably needs to be lower, and handling is particularly difficult during rush hours when I need to slalom between cars. Starting is also a bit tricky as the bike tends to wheelie.
It obviously needs some practice in order to handle the bikeseat properly!
Overall conclusion is to carry a passenger, a long wheelbase frame is the best option, and smaller wheels will probably be much more appreciated.