Usually this is caused by the two parts not being in exact rhythmic unison for the entirety of the phrase, forcing the music onto up-stem and down-stem voices. This can be caused by several things: a playing technique or a bit of text that’s only in one of the parts, or a dynamic that’s rhythmically offset by 1/16. Also, Dorico looks at the entire phrase, meaning “everything between two rests”. Sure, at the start you have unison whole notes, but perhaps the lines differ at the end of the phrase? Without seeing your score, it’s impossible to know what’s causing this.
Also, the condensing feature in 3.0 is still far from fully developed, finer-grained controls have been promised in future updates. There is apparently an update imminent “before the end of the year”, too but we don’t know yet what will be in it.