Normal and cue notes

Hi - I’m a Cubase 9.5 user.
Need help with when placing two notes together (duet), wanting one as normal, the bottom as cue. Most times when I turn the bottom into cue, it changes the whole stems and beams into cue as well, but I want these to stay as ‘normal’ size. This doesn’t always happen but don’t know why it happening or how to fix it or control it. thanks