The STR rental communities that I know of that have a shuttle service use the same company - Maingate Transportation. Those communities are Emerald Island, Windsor Palms and I would guess Windsor Hills too. Perhaps other communities in the area use them also.
They are reliable within the service that they offer, but the standard schedule is (dependent on the time of year) two pick ups in the morning and two in the evening.
I think the shuttle service is great for families with older, independent, teenagers who may want to go to universal whilst the rest go to another park
They also offer a private shuttle (or taxi) which can be more flexible for your needs.
If you think you will need to have two cars, then consider getting two smaller vehicles.
-Kay