Adding notes over other notes

I am having a weird issue - I guess I have some setting turned on so I can't add notes on the same stave at a time when another note is played. What I would like to do is to play two quarter notes while a dotted half note is also playing (see attached example).

Is this possible in MuseScore? The program won't let me add the half notes anywhere in the bar, except at the very start of it. And if I do it the other way around, the half notes will delete my quarter notes.

Thanks in advance