Is it one single purchase for 5 tickets? if so, you'll have to cancel the whole purchase and buy three new ones for the ones that still want to go to Together. Then buy two separate tickets for Carl Cox. Don't forget you can't do both purchases on the same credit card as the owner must be present at the door and must enter the club as well.
Also, mind that, if you bought early bird tickets for Together (which were priced at 23€ and on sale until mid-April), these tickets are non-refundable.