Yes it will prevent you from using a S2. If you list on the same PNR so you will clear together it downgrades to the lowest priority for everyone on that listing. You will be a S4. A good agent can split the PNR at the gate and switch you to a S2 but don’t count on it right before departure. Do not under any circumstances try and cancel the listing and rebook separately. If it’s less than 75 minutes to departure you will not be going at all. The best option is two different listings. Don’t try and tell the agent you’re only going if your S4 friend gets on. Just wait and see if he clears also.