The two parks have such different climates that there's no one ideal time to visit both. That said, going in mid- to late September rates to be the best compromise.
Northern California weather typically stays warm through September. You can expect pleasant daytime temperatures (25C) in Yosemite Valley. It'll be cooler in the Tioga Pass area (15-20C daytime) but not cold enough for snow or road closures to be a problem. The main drawback is that several of Yosemite's waterfalls are dry at this time of year.
In late September it'll still be hot in Death Valley but not brutally so. Use common sense about the weather (e.g., drink lots of water) and you'll be fine.
BTW, skip Groveland. Some people think it's a charming little village, but really it's just a gateway town, over an hour from Yosemite Valley, with low quality inns at high prices. Spend the extra coin to stay in the park or somewhere much closer.