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This article provides instructions on how to do this:
https://www.gardeningknowhow.com/edible/fruits/olive/how-to-plant-olive-pits.htm

I didn't find any information on double blooming. Olive trees bloom in the spring and often produce less fruit in alternating years

You are in zone 8b, which should be good for growing olive trees. The most common reason for this is if you've experienced heavy rains. Since many olive trees pollinate through the transfer of pollen from wind, if their environment has been too wet it can hamper the process. The other reason could be that you have non-fruit bearing olive trees. But remember that olives often take up to 12 years to become mature enough to bear fruit, so patience might be a key factor.Here's a good article: https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-8/zone-8-olive-trees.htm

You may not be able to save your tree, depending upon how deep the damage is. You will need to consult with a certified arborist to be sure. This article discusses how to tell if the tree has died: https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-scratch-test.htm
You can try trimming away the dead branches, leaves and buds, but pruning it severely now will cause more stress if it's still alive.

It may be too old now to remove a trunk. Here is more:

http://cesonoma.ucanr.edu/files/27176.pdf

This article should help with general pruning:
https://www.gardeningknowhow.com/edible/fruits/olive/pruning-olive-trees.htm

The first thing will be to test the soil for deficiencies and pH swings. These articles will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

The next thing to consider will be how long the trees have been planted. Generally, you can expect at least 2 years before the tree establishes, fully. After then, if they still haven't produced, then you can move on to diseases. Generally, there will be signs of disease that are very noticeable.Browning leaves, black spots, or other symptoms will indicate disease.

One last thing to consider will be that you make sure that the tree is not an ornamental olive. These will never bear fruit to an amount that will be enough to eat.

While you sort through the issues one at a time, here are some articles that will help:

https://www.gardeningknowhow.com/edible/fruits/olive/growing-a-fruitless-olive-tree.htm

https://www.gardeningknowhow.com/edible/fruits/olive/olive-tree-care-information.htm