No problem at all. In fact if they happen to be paying out of pocket for something (say a kid's meal or maybe a cheap CS breakfast at one of the value resorts (where DDE is accepted)-that they don't want to waste one of their CS meals on) you could put it on your DDE and save them 20%. If they want to share a TS meal and maybe just order an extra side, you could order the side for them and get 20% off for them. They could just pay you back in cash later. Tell the waiter you have the DDE card and they have MYW dining, so you will need two checks.
I am using MYW dining when I go in Sept, but also have the DDE card which I can use for any extra sit down meals or for breakfasts or snacks at POP.