I would say go for it, they are different enough in shape so they shouldn't fight. I have a hippo, yellow, and powder brown in my 240 and it still doesn't seem like enough room for them. I had my hippo and yellow together in my 90 gallon with about 100# of live rock and they were fine. Long term though I think the Hippo tang will definately need more room. Keep up on water changes and feed them sushi nori from the oriental foods section of the grocery store. My tangs love it and are healthy and fat with great color(of course feed them other staple foods as well. Strong water movement and alot of liverock are key factors for thriving tangs. Good Luck
I wouldn't. A blue tang grows too large for your tank also and cramped spaces are only going to increase aggressiveness, not to mention the toll on your water quality with two constant eaters and poopers.