The Arena DLC got a co-op mode (local) in the last update. Just hit Y or space to join after starting a run. The score will be posted according to player number.
We also increased the enemy power in higher arena levels. The scores were multiplied by 3 so you should be able to score higher with new runs.
A bunch of other smaller fixes were included as well. The main game can now spawn "credit crates" to lower the amount of credits to draw in super high Overseer levels.
Full patch notes here: http://steamcommunity.com/app/428750/discussions/2/142261352648589383/