If the worry of flutter has you too concerned, you can add a fix that I do on my planes. The problem is that with many/most of todays ARF's, one of the fixes isn't as easy to do as it is on a kit built plane.

1. IF YOU HAVE ACCESS to the rear internal area of the fuslage, you can make a simple brass sleeve to support the rear portion of the DB rod just an inch or two in from of where the "Y" wire pushrods exit the rod end. I don't remember the exact size of brass tubing I used, but it was a standard size carried in the hoppy shop. Anyway, I cut a short length of the brass tube (about 1 inch long) to make a bushing that will just slide over the rod. I found that after I cut the bushing to length, it helped if I used a small round file to very slightly work over the inside edge of the bushing on each end so it doesn't have an "edge" that can dig into the rod. Then I slide the bushing over the rod BEFORE (IMPORTANT) I assembe the wire ends of the Dave Brown assembly (otherwise the bushing will not be able to be placed on the rod afterwards. Then I go ahead and put the Dave Brown pushrod into the plane and hook it up to the servo and elevators. Once in place, I then slide the brass bushing back until it is just an inch or so ahead of the "Y" end of the pushrod. Make sure to allow enough room for the rod to move fore and aft without the "Y" end coming into contact with the bushing. Then I CAREFULLY glue a wood cross brace (usually 1/4" or a little larger HARD balsa is fine) between the two fuselage sides, and DIRECTLY under/against the bushing. Once the glue for the cross brace is set, I then CAREFULLY glue the bushing to the cross brace (it may help to roughen up the outside of the bushing with coarse sandpaper first) with epoxy. You must be very careful NOT to get any glue on the rod itself, or nothing will move afterwards!
As long as the overall length of the rod is at least 1.5 feet or more long, and you make the bushing no more than about an inch long, there should be just enough play to prevent any binding from the slight side to side movement at the servo end as the servo wheel rotates, but the bushing will hold/center the rear end of the pushrod and eliminate almost all of the differential movement you mention with your split elevator halves.

2. If your plane is an ARF, or you already have sealed up the fuse so access is no longer possible, there is another fix that should at least help. You can take short lengths of plastic "Nyrod" type tubes/guides and side them over each of the wire "Y"'s where they exit the fuselage (you will have to remove your clevises first) and through the exit slots in the fuse sides. Then glue these plastic "guide tubes securely into the fuselage sides.
The trick here is to make the tube/guides just long enough that they leave at least the last couple inches or so of the end of the wire rods exposed so nothing interfers with the clevises, and also that they extend into the fuslage as far as possible without interfering with the main pushrod at the "Y". If they are glued in too far into the fuselage, full travel of the pushrod will be reduced when it runs into the guides. However, if they are not glued in as far along the wire "Y" rods as possible, then more slop will result in the individual elevator movement.
Once the glue sets, the plastic "guide tubes" through the fuselage sides will help reduce the side to side movement of the back end of the main pushrod (that is responsible for your differential movement of the elevators).

Phew...hope this helps!

Lee