Pro mužský a ženský rod se použije vždy oni, mužský rod má přednost. Pán a paní jsou tedy "oni".

What is the difference between "oni" "ony" and "ona" in čeština?

According to my workbook, there are three words in Czech language for "they".
For example, pán+pán=oni
paní+paní=ony
křeslo+křeslo=ona
But which word should I use for pán+paní for example?