Equally bad at something with both hands.
I'm ambisinistrous at Wii bowling; I suck on both sides.
Both left handed; dominant left, but uses right hand for particular tasks. Contrary to notion that it means bad with both hands, the word "bad" is not in there. It derives from Latin. Ambi=Both and sinister=Left. The opposite of ambisinistrous is ambidextrous which means both right handed. It does not mean good with both hands. All left handed people are forced to be ambisinistrous because in a right handed world, there are many things that are not made for left handed people.
I am ambisinistrous because I eat and write with my left hand, but I play the guitar with my right hand.