Though I’ve been in the OP’s position before, I agree with others that the best solution would be to sell your present cables and invest that amount in a nice set of cables of the proper length, hook ‘em up, let them cook a bit and call it a day. That being said, as someone who prefers to do things the hard way, see if the manufacturer of your cables will sell you a single terminated cable of a length double what you need. Cut it in half, and, splice each half to one end of each of your present cables just short of the termination already there. Preserve as much of the inner goodness as possible, and finish it off by sealing the splice with a proper girdle of heat shrink. Be sure to slide that heat shrink on before soldering, you don’t want an “Oh Schiit!” moment! Maybe less expensive would be if the manufacturer has a 3-4’ scrap of unterminated cable they’d sell cheap. But that would mean 2 splices (and twice the work) on each cable, to preserve the existing terminals.
