How much? 0 pounds per 1,000 square feet
When? Never

What is the problem you want to fix? No matter what your problem is, those two products are not the answer.
Where in LA are you? -be specific. Don't need your address but the town would be very helpful. Conditions in San Pedro are much different from conditions in West Covina.

What do you use the lawn for? Sports, pets, just looking beautiful?

As long as you are answering questions, I may as well ask the rest of them...

How often do you water and for how long?
How high/low do you mow and how often?
How often do you fertilize and with what?
Have you used any herbicide, insecticide, or fungicide in the last year?
Is shade a factor in any of the areas you are concerned about?

Is it 'death grass' because it is always dying? Does it spread at all?

You're watering exactly right, so that's really most of the way to a great lawn.

The bermuda and St Augustine will compete with each other in a good way. Unless you want to have the Yard of the Month, I would leave it like that. St Aug will thrive in the shade and either will thrive in the sun. If you mow it as low as 1 inch high, then the yard will always look like a bermuda lawn with weeds. If you mow it at 4 inches high, eventually it will all look like a St Augustine lawn. The reason is the St Augustine's wider blades will shade out the bermuda very gradually. But if you want to favor the St Augustine, you must NEVER forget to water. Sure you can skip once but don't skip twice. St Augustine does not go dormant when it dries out, it goes directly to dead. If that happens, you will have a full bermuda lawn in a few weeks. Since you have shade some of the St Aug in the shade will survive to fight back. You can run that battle back and forth for years if you want. I've done it...kinda fun for a lawn nerd.

If you want to favor the bermuda, then search the Internet for the Bermuda Bible and follow that. It requires lots of fertilizer all year long (since you live where it never freezes). If you want to favor the St Aug, you can get away with fertilizing 3x per year (Memorial Day, Labor Day, and Thanksgiving).

If you really want it to jump out at you (you said it was not spreading), you can try fertilizing with an organic fertilizer like alfalfa pellets (from a feed store). Apply 20 pound of pellets per 1,000 square feet. If you want it to really go, do that every month for the rest of the season. A 50-pound bag of alfalfa should cost around $12. It also comes as rabbit or chinchilla food. The reason you can use organics in the summer and not synthetic (Scott's) is that the organic does not feed the plants directly. Organic fertilizers are made from grains which feed the bacteria and fungi in the soil. Then the soil microbes feed the grass. By going through all the biology, the grass doesn't get burned by the salty synthetics. It works! Here is a little motivational picture I clipped from GardenWeb last year.

That is a zoysia lawn. Note the improved color, density, and growth. That was after 3 weeks. It takes a little time for the organics to work the first time. You'll get the same results, I promise.

What else???