I see a lot of Type 055 and Type 052C/052D Destroyer threads.
1) From my understanding, the Type 055 is independent from the Type 052D (not an upgrade), and both will be built simultaneously. Is this correct?
That is correct.
2) If so, what military value is there from building both at the same time? Is the Type 055 basically a large destroyer, and the type 052D is a medium/small destroyer?
One is a lot more expensive than the other.
One has a lot of new technologies that has yet to be proven in years of operational use. The other is a set of operational proven technologies that already has a number of years of deployment, highly tested and refined. Confucius says, Do not put all your eggs in one basket.
This is only a temporary situation. Production volumes will decrease the price of the Type 055, and as time goes by, numbers go up, the Type 055 will go from the unproven to the proven. I don't believe in rushing production of the Type 055 too fast. They should let the first one or the first few run around, do missions, to help them reveal their operational shortcomings, so they will be fixed, and incorporated into the design level when building more of their sisters.
There was a time this situation was reversed, with the Type 054A the mass production, more affordable model with proven technologies, while the Type 052C/D was the new kid in the block, that is very expensive to make with new, unproven technologies, especially with its large active phase array radars. People thought, they were only going to build two, maybe six at most, of these ships. They did build only six Type 052C, but the mass production of the 052D in much greater numbers wasn't expected.
Maybe years from now, the situation is reversed, the Type 055 will be the small, budget ship with proven technologies that is being mass produced like hamburgers, and there is an even bigger monster on the block that is the new gourmet on the menu. The one constant curve in modern warship trends is that surface warships are constantly growing bigger --- note how the concept of the frigate and destroyer keeps growing in size.