I'm wanting to modify a four star 60 for Pull Pull on rudder. This is my second four star and I increased the rudder area by about 30%.

My question is, What type of control horns on the rudder should I use? Do I put two of the standard white plastic ones on at different locations on the sides, or is there some that bolt across from each other somehow?

What ever type you use, they need to be in line with each other and equal.

If they are in line, I thing the elevator horn will interfere. Do they have to be across from one another?

Not only should they be even, but they need to be in line with the pivot point on the rudder. You also have to make sure that the servo pivot points are inline with the centre of the servo. This helps stops getting one tight and/or loose Push pull wire.

As long at the CtC is the same on the servo horn and the rudder horn the wires don't have to be in a straight line to stay tight. They don't even have to be the same length. Look at one of the Morris profile planes to see this. The tubes are ran inside the fuse with the servos in the wing and it's no problem.