Cut them into shorter sections, to avoid creating a stress point on the sword.You can also use two cores for your swords. You won't need much counterweigting at all then. What counter weight you use can be placed on the side of the handle so that you don't have stress points in the direction of your swing.
Two cores is an extra point of failure, worse weighting, heavier, more nasty glues, etc. I could go on for hours about why double coring is a horrible idea, but I'll leave it short.
OP: I love to do my counterweights (on .524 kitespar prog-build swords, most recently) out of two-and-a-half to three-inch-long pieces (for the reason Magnus stated: avoiding stress concentrations by way of changin the flexural modulus of the core too much all at once) of ~1/4" round steel barstock. I do two-wide up each blade edge, producing a lovely roughly rectangular cross-section handle area. Leave about 3/16" between the ends of the 3" bars to give them room to flex without touching and I recommend attaching them to the core with a layer of double-sided carpet tape, to add a little extra flexural padding between the steel and the core. A layer of hockey tape under the carpet tape doesn't hurt, either. Once the stock is on, just add a few bands of hockey tape to secure it, then another layer of carpet tape and wind twine, 2mm macrame cord, or other small rope around the handle, for a little softness over the steel and a good texture. I typically finish with a final layer of hockey tape over that, myself.