Do you realize how many miles that is and how different the climactic conditions are in Yosemite and Death Valley in March? Also note that to get to Yosemite from Death Valley, you'll have to go back via Mojave in the south or SR 89 in the north, as most Sierra passes are closed in winter. Probably 10 hours of driving.

I wouldn't even recommend a convertible for Death Valley, btw. An SUV would be a better rental. Carry chains, and if your car isn't 4WD with snow tires, you might well have to use the chains. I don't think a lot of visitors realize that March is typically the month in the Sierra with the highest snowfall.