Sickle-cell trait : Why is only one type of beta chain found in any one hemoglobin molecule?
For one specific hemoglobin molecule, yes it's just one type of beta chain.  In the case of a heterozygote, this hemoglobin molecule could be expressing from the bS gene OR the normal beta gene.  Let's call it a 50/50 chance for each hemoglobin to be expressing bS.  Because RBCs have a ton of hemoglobin in them, this 50/50 or so expression of bS wouldn't be enough to cause RBC sickling and disease, and it also helps explain the heterozygote advantage vs malaria.