Alright, so, it sat in the back of my mind since having the Festive Pack for free worked this way, that it might in fact correlate to a problem that just plagues Split-Screening altogether.
We recently purchased the Onslaught pack on both of our accounts on the PS3 but the only person that can access the content is whoever is signed in under the 1st player controller. ( By content I should elaborate that so far, this applies to the guns, Festive camo and reticle, etc... The maps may still be accessible, I'll have to check. )
Despite having spent the extra $15 to get it on another account, 2nd player can't access the items in the pack.
Is this a known quantity? Are there any fixes in the work for this? Is this just a problem that's being ignored altogether because there is no fix? etc...