I've answered my own question. These codes can be combined...
I assure you that your case is the exception rather than the rule. I've been intimately involved with Live Nation codes for a few years now and this is the first time I've heard of anyone who's been able to stack multiple codes in the same transaction, be it for tickets
or merchandise.
In fact, after a little digging I looked at the
Live Nation® Concert Cash® Code Terms of Use where in part it says:
One Code per transaction.