The "suggested" tip of 18% is automatically added to any party of six or more. As stated, this is getting to be the norm for any restaurant, not just Disney World. It doesn't matter if your bill is split or if yo put down two different credit cards, the "suggested" tip will be clearly marked.
Now, if you have received less than average service, then by all means ask to speak to a manger and have the tip removed. And if you think you received above average service, leave more (you can just write in more on the bill or leave cash).
One reason that this policy is very good. If you have received less than average service and want the tip removed, the manager will ask you why. And then you will tell him/her. And then the server will immediately know. And the behavior you found to be less than average will be corrected immediately. Instead of you walking away and being mad.
In the OP's original situation, there are two people meeting four more. One or the other group might not even believe in tipping. So that server might get less just because of the beliefs of that entire party. In another situation, I believe there were going to be three groups of two. All getting the same service but there could be different amounts left for a tip. So what is the server going to think? He/she messed up for two people but not for all six?
Personally, let the United States finally pay their restaurant servers at least minimum wage and do totally away with the tipping. Problem solved for everyone and no more debates.