If there is a back-door to either AI empire, then send a small strike force to harass it. The AI will send his main fleet to intercept, leaving his front border completely undefended. You can now move in, capture some territory, and have it fortified before the AI gets back. With proper scouting and cat-and-mouse play, I've reduced an AI down to his homeworld while his fleet just travelled back and forth between my two smaller fleets.
Split fleets works wonders against the AI, because it always keeps one massive "fleet of death". Two smaller fleets that can fall back to a line of static defense will absolutely destroy the AI, because it will always leave one of its borders undefended, and so long as you're careful to scout and know where his fleet is so it doesn't catch you by surprise he'll just spend all his time running around while you ravage his empire.
Another nice strategy is to plot down some PJI's, then retreat your fleet away from the front lines. The AI will jump into the system, thinking it's "undefended" even though your main fleet is only one phase jump away. Now jump back in and slaughter him. The AI is stupid, never learns, and is woefully unadaptive. Use this to your advantage 