You need the tails long enough so that when you have tied them off on corners of the transom, they enter the block on the boom-end separately(when the mainsheet is pulled in tight for beating in medium winds)
If they join before they reach the block, you will lose the benefit of the split tail - the leeward tail will restrict the boom position & the boom will not be in the centre of the boat. This will reduce the ability of the boat to point high
Hope this helps
The tails should be attached about 10inches forward of the transom, they should also be attached together with a bit of elastic, about a foot into each tail. That way the mainsheet does not get caught around the transom when you gybe.