You can already d your dual-purpose idea
how to:
1.) setup both PHH/HH and AA guns on your R or T slots
2.) load HH shells in their A locker
3.) Load AA shells in the B locker
4.) press Y to switch from ASW mode to AA mode and vice versa
It is not very effective because i would think you would need at least 5 guns on each side 2 for PHH/HH and 3 for AA because anything less than 6 AA shells in 1 salvo is ineffective