I only know one way to do this... Maybe there is more of this, who knows...?
My way is to putting two circles, one on a tick and the other one on an other tick. Then, I go on the timeline, and I move the circle I want to put on the same tick as the other circle, and that's done. That might be quite confusing, but it's quite hard to explain this way.
I think you can also do it by editing the .osu file, but I don't know how to do this, and I don't want to say dumb stuffs, so I will not say things about it ! o:
Have a good day