Is it necessary?

I finally -- after waiting literally decades! -- got a pair of
blueberry bushes that had been adapted to need much less winter chill,
so can be grown in So. Calif coastal.

The "O'Neal" bush is loaded with big fat delicious blueberries

Its companion, "Sunshine", is also loaded -- but with tiny, miserable-
looking berries that are drying up by the day.
I am returning it to the nursery.

Questions:

1. Is "cross-fertilization" necesssary? Some of the sites I visited
said yes; others said (AFAIK) that O'Neal was self-pollinating but
would be better with cross.

2. If so, why can't it be between same varieties, rather than across
varieties?

I am dizzy trying to evaluate the many varieties listed on the sites I
visited. My #1,2,3, and so on requirement is FLAVOR. Prefer more
tangy than mild.

Your input appreciated -- keeping in mind this is So. Calif. Coastal.

TIA

HB