Do one party for both children, ours are around the same days apart and at such a young age, it doesn't matter. When they are teens and have their own sets of friends, ee will do separate parties then.
From personal experience, have ONE party. Do NOT cater to people who can't get along, even if it is justified. They can suck it up for two hours in order to celebrate their kids and if they can't then they have bigger issues. Everyone can learn to smile and be cordial or ignore each other at special occasion events. If you have to talk with people that you expect everyone to be on their best behavior at the party like another mom suggested, then you should do that.
If you make a separate little party just this one time, then it will be expected every time and it will only turn out to be a big stinking hassle for the rest of the kid's party days (and Christmas and Easter and Halloween and any other time you have a get together for the kids).
My sister does this so our mom and mil won't have to go to the same party b/c they don't like each other... frankly it gets ridiculous to have to throw 2 parties just to make one or two people happy, and she can't afford it and is extremely busy herself, but she is finally learning to put her foot down and stop it.