The fruit will develop bigger if you prune off some fruit not vines. If the vines are too big, you could prune off both. But pruning the vines and not reducing fruit number will likely give small possibly low quality fruit.

Black Diamonds are big melons. You should allow 30 sq ft of foliage per melon if you want 30 lb fruits.

If you want to prune in order to reduce the size of the plant, you can surely do that. Greenhouse melon producers have their melons climb up poles or trellises and they generally cut off all secondary vines except for one. So, each plant ends up with the main vine and a secondary one. Of course, the more vine you cut off, the more fruit you lose, so choose the vines to be pruned wisely. Don't just cut off the tips, cut entire vines.

If you want to prune to hasten fruit development and to improve fruit quality, you could remove some of the fruit and leave the rest.

In any case, don't cut the main vine growing tip. And if you do prune, be careful not to disturb the vines. Watermelon doesn't like to be fiddled with.

Thanks everyone... think I will leave Nature alone in the melon patch & see what SHE does with all the vines!

BTW: how are 'seedless' watermelons developed & how are the seeds that are sold developed x seedless melons? Just curious!!

Are these 'seeds' hybrid?

Seedless watermelons are sterile hybrids and are grown with a pollinator plant so they can produce fruit. In your case, I assume your Black Diamond will pollinate your yellow seedless variety. Good luck with them!