This will be spread over my 12K sq ft back yard only... so if I am figuring correctly, that would be just over 1.25 lb per k.
Do you have a particular amount of nitrogen you're seeking to put down per 1k? Otherwise, I'm going to assume you're wanting 1lb of N per 1k sqft of lawn. With that being said, 1lb of N a month is normal for Bermuda but sounds heavy handed for St Aug so you probably want to put down less. Without further ado, onto the math.
If you're wanting to put down 1lb of N per 1k sqft, you can spread that entire bag of 33-0-0 (assuming 25lb) and that would get you 0.63lb of N per 1k sqft. If it's 30lb of 33-0-0, that gets you to 0.76lb of N per 1k sqft.
So, use the entire bag of 33-0-0, and depend on how much of that you have you are going to use the all purpose fertilizer to make up the difference in nitrogen.
If you have 25lb of 33-0-0, use 37lbs of the 13-13-13
If you have 30lb of 33-0-0, use 24lbs of the 13-13-13
37lb * .13 = 4.81lb of N total/ 13 ksqft=0.37lb of N per 1k from all purpose
24lb * .13 = 3.12lb of N total/ 13 ksqft=0.24lb of N per 1k from all purpose