I'll just re-iterate what's been said... there's no other explanation really, apart from jealousy.

The 2 girls feel like you've wrecked the circle of friends, which is only true if they LET it be ruined. True, things wont be the same.. but if they were true friends, they'd get over it and adjust to the changes.

Talk to them both individually and explain the situation to them, don't get them together as they'll back eachother up and you won't have a chance.

If you still cant reason with them like that, then you've just got to leave it up to them if they want to accept it.. you cant force acceptation on them.

Hope this helps,
Nick