The key is to make sure you have sufficient resources to accompish your goals on any front you have.
If you want to attack a Civ, make sure you assemble a sufficient force to be decisive. If you're going to sabotage a city to destroy it's walls, don't go in with 2 diplomats and hope they hit the walls. Make sure you have 7-10 diplomats in position to sabotage the city in one turn. Don't go after a city with one or two offensive units, Go with enough units to take out all defenders. And make sure you have defenders ready to move in and secure the city once it falls. If that means you can't devote resources to a second front, then don't actively fight the second front. If you have the resources to fight both fronts with decisive force, then by all means, do so.
Note that I said you may not want to actively fight the second front. Just because you are not taking the offensive to the second front, doesn't mean you must sue for peace either (although sometimes that is prudent). If you hold sufficient defensive positions, you can keep the second front at war and just let the enemy throw their sheilds away at your defenses while you concentrate the bulk of your forces on the "active" front.
It's all a matter of your sitaution, and your goals.
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DEATH awaits you all...with nasty, big, pointy teeth.