both sides with one flange/bottle?

I have to pump once a day at work for my 4.5 month sweet boy (EBF). I try to get 4 or 5 oz during this session, and fortunately I can manage that just on one side most days. On days that I can only get 2 or 3 oz from one side, is it sanitary to use the same flange and switch to the other side to fill the bottle? Should I switch out the parts with clean ones before I move to the other side?