I know people who already weigh 2x times as much as they should and they don't have any problems getting around. It would be like serious weight training with weights on all of the time. I think 2X gravity would be livable, though it would either strain your heart, or you would have really good cardio health because of it!
Weighing twice as much as you should isn't the same as the increased acceleration of higher gravity, which means impacts are more severe, etc.
At the very least, it would be an issue going immediately from Earth to a 2G planet. People who weigh 320 pounds didn't gain the surplus 160 overnight. Their muscles were conditioned to accept the extra weight incrementally.
Forgetting the other circulatory effects for a moment, I wonder what effects even comparatively slight increases in gravity (2, 3Gs) for extended periods would have on air pressure. Would going back immediately to a 1G environment cause problems, i.e. bends and such?