I have not tried this particular combination but I typically do use a variety of ingredients together (fruit, herbs, spices, flowers). Occasionally, I do them in separate batches and blend afterwards - but that's rare. Here's my thinking/approach, fwiw.
First, I tend not to ferment only juice since the pulp will add body and mouthfeel depending on what I am fermenting. There are certainly cases where juice-only is an option, like apple, sometimes orange, or perhaps making a tea to add to a blend. Since yours is primary grapes and the fruit is adding something extra, this might be a case for the nectarine juice route fermenting separate or adding to the secondary as needed for the amount of fruit flavor you're looking for.
Second, I, personally, tend to ferment all at once - mostly out of simplicity. I am happy to experiment and accept what I get. Usually it works and when it doesn't quite make it, it's not off by so much that I care. I may make note and adjust the next time. If I were trying to perfect a combination, then, for sure I'd keep separate in initially (but might aggregate once I understood the proportions).
In the occasional times I do ferment in batches, it's because I am working with a strong flavor that might overpower. Rosemary is one such example. The difference between a cup and a half or quarter cup can be significant in the end product. And this certainly varies with what I am pairing with or from batch to batch of the same pairing. Safer to keep that separate until the last minute.
I am guessing you're working in larger batches from what you described but trying to juggle multiple small amounts will produce more waste unless you want to simply bottle the small amount of the leftovers that did not make the blend. Additionally, fermenting a couple/few small batches is always more cumbersome.
One final thought is that you can avoid the whole parallel or aggregated ferment question altogether and make an FPAC that you can flavor the wine with later. If you're looking for upping the volume more than a smidge, then this might not be the best approach but you might be able to stretch it a bit and use the FPAC to add more flavor to a weaker grape-only wine. << To some that might be heresy. I'm just saying it's an option.
Personally, I think you have options that are not significantly different. If you are looking for simplicity and willing to accept what you get, doing in a single batch is the way I'd go. But trust your gut.
