You can definitely do both in one day. Wifey and I did it in 2005 and had a blast. We started with Xunantunich early, ate a casual lunch in San Ignacio and then headed back toward Belmopan to catch the cave tubing. It is a long day but I didn't find it too jam-packed at all.
I do agree that if you can stay at Jaguar Paw or another recommended place on the mainland and spend several days, it is well worth it.
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Say it 5-times fast: "I buy my BBQ and Belikins on the beach at BCs!"