Pulmozime works to break up the mucus in the lungs, when it gets stuck there it gets dna spewed all over it from your body trying to get rid of it making even more sticky, the pulmozime breaks that up.
HS works to just make you cough up the junk that is already loose so the two working together is most productive, thats why they tell you to do albuterol first to get the passageways open, then pulmozime to break it up and then HS to get it out and then the vest to break loose/cough up anything else it can.
I wouldn't recomend skipping one or the other because in the long run it won't do you any good even if you can't feel the difference it makes. If you insist on skipping one or the other skip the HS because it can sort of be replaced by the vest or equivolent. Before you decide to skip it though, use it for a couple of weeks to see if you think it's something you can afford to not do