Assuming you're talking about the "mini" ones, the ones that don't get much larger than a quarter, just take a knife and slice it in half through the mouth trying to keep equal amounts of the oral and pedal discs with each half.
If you're talking about the "maxi" ones, the ones that get up to about 6" in diameter, I'd do it the same way... but I don't of anyone who has manually split the larger ones so you'd be treading new ground without any guarantees it is likely to succeed.
- Mark