K.C.
I would not rent the stroller for the 6 month old, it's more of a cart than a stroller.
I've never been to sea world CA, but I live about 20 min from Sea World Orlando and I"m sure its the same stroller, it's low to the ground and just has an area scooped out for the kids to sit it. I would bring a single stroller for your baby and then if you needed to I would rent the single for for 4 year old and hopefully you will have someone with you to push the other stroller. The 4 year old will probably be to excited to sit most of the time anyway. There are also tons of show at sea world so he will be sitting during those anway and on all the rides and in the playplace he is not going to want to sit down he will want to walk around.
What do you do with both kids when you are at a shopping mall or other crowded place? I would use the same procedure that you use for those occasions, just talk to your 4 year old about holding your hand or holding on to the stroller so you can see him.
I would not go buy a double stroller just for this once! To expensive and not enough use, unless you can get one cheep on craigslist or somewhere like that.