In the spirit of what FDXLAG is talking about, and if you want to fly both the 26th and 27th or will settle for either one of them if both aren't available, you need to make three entries:

1. trip X/26DEC, Trip Y/27DEC
2. trip X/26DEC
3. trip Y/27Dec

That way you can get both if available or one or the other if one is taken. Good luck.