Having two WLC with licenses can always be used for redundancy. There is no need to purchase an WLC2504 HA sku since you already have two WLC2504 with licenses unless you want a backup WLC for either of the ones you have now or backup of both. Like Sandeep mentioned, the 2504 only support N+1 which is really a backup WLC. With your setup of two WLC with 15, you can split the load and have 7 AP's on one and 8 AP's on the other or all 15 on one and the other for backup.
Typically with one 2504 with license and the other 2504 being an HA sku, all the APs would be on the licensed WLC and the HA sku would be the backup. The HA sku doesn't need any license and that is why it makes a cheaper solution for a backup WLC. You can make one of your WLC act as an HA sku, but you would meet a 50 AP license which doesn't make sense since it will cost you more probably.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...