Shorter note in unison overrides length in playback

I have a two-voice passage on the same staff, in which one voice moves in quarter notes while the other in eigth notes (quavers). When both voices reach a unison, the eigth note duration overrides the quarter note in playback.
I've solved this problem in an imperfect way: I've replaced the quaver in the unison with an invisible silence, then extended the beam, then added a vertical line to simulate the stem. It looks right and also plays right. The problem is that the vertical line is not tied to the notehead nor to the beam, so any change in layout is not adaptive and must be fixed manually.
I've also tried to place an invisible tie starting at the shorter note, this works but the tie may extend along several measures until the same pitch is found (if ever!). While the tie will not print, it shows in light gray and may be visually disturbing while editing.
Is there any way to circumvent the underlying playback issue?