Hmmmmmm, Katiewatie encapsulates the problem with setting such store by a very managed day. Her wedding has become a source of malcontent (people she will never forgive) for the rest of her life because it wasn't as she had built it up to be beforehand. That is sad, mostly for her.
Yes, my wedding day went wrong - cooker broke, hairdresser was rubbish (I had to rush off and rewash and dry it), my bridesmaid never let me get to the mirror to do my make up because she was so bothered about her own. But do you know what? I couldn't care less...my family and friends were there to celebrate a happy event. We all liked each other, danced, ate, drank wine and had a great time. My bridesmaid is STILL one of my oldest and best friends, and I love her dearly for all her foibles.
OP, this comment:
"Better to offend them than ruin my big day."
Are you sure you have your values straight?
You are hypothetically writing off 2 of your closest friends (only 20 guests and that includes family). You have predetermined to write off their friendship in order to have your "Big Day" exactly how you want it (??would that really include having a row and losing 2 of your friends?) And they haven't even done anything wrong yet!
You don't want DC at your wedding (fine, it's your party). You think that these 2 may bring their DC anyway and if they do you are planning to have a stand up row at your own wedding??? If you like them and want to be friends and know that they want to bring their DC so much that they would ignore the invitation, then ASK THE DC TOO. If you don't care and don't like them, then don't ask them to the wedding at all, as they are not your friends. You don't SEEM to like them as you are already assuming that they might be rude enough to turn up with uninvited guests.
Yes, it is YOUR big day but it is your life that you should consider more, and how you treat your friends will have a large effect on how happy that life is.