two best friends. one party. help?

[Partyy machinee =]]]

Alright so i love both my best friends to death. but i wish they loved eachother!
they're named are Natalia and Kate
us three used to be best friends but recently they decided they dont like eachother anymore. they can't STAND eachother.
they sit on the opposite side lunch tables and i hate it. because if i talk to one the other one gets jealous etc. etc.
BUT THATS NOT MY QUESTIONN.
my question is that.
well im having a party in a couple of weeks. i've asked both of them to go because they're my best friends. they're both available and want to go and everything, which is good.
Natalia is being the bigger person and is saying she will go even if kate is going because its my party, not hers and its not her choice on who to invite. she says she will just ignore kate and try to enjoy herself.
now kate is being unreasonable. she refuses to go unless i dis-invite Natalia. Everyone agrees that she is being unreasonable.
what should i do? i really want to invite both my best friends!