Vyshka:
There is not currently any way to get an AI to break an Alliance. The best thing to do is defeat the weaker partner and force a separate peace. After a couple of turns, the stronger partner will invoke the Alliance and make it re-declare war. Repeat, this time getting a planet or all available techs, trade goods, etc. in the deal. Eventually, the weaker partner will be reduced to its last planet and forced out of the game in the last deal you take for it.
In the meantime, fight a Fabian delaying action against the stronger Ally. You will not be able to settle peace with it while it has Allies still in the war. It will accept your offer and then re-declare war 2-3 turns later due to the Alliance. So (much like the Italians and Romanians on the Eastern Front), target the weaker of the 2 allies.
Apparently, Allies take a diplomatic penalty with each other if they break Alliance by not going to war. (To see this in effect, refuse to declare war when one of your Allies is attacked. The Alliance will be immediately broken, and your relations will be downgraded to "close".)
#2 IG,
Pre-requisites are:
a) Alliance tech researched;

Diplomatic relations at "Close".
You may then offer an alliance on the diplomatic options screen. Sometimes the AI will even offer you stuff (techs, money) for it.
~SDC~