You need to make a divider which seperates the inside of the can into 2 halves, one half for each hose. This is best made from thin alloy...
At the bottom of the divider you should drill some holes for the air to go though, note these holes should be at the bottom as it forces the air to go through the stocking which is the next bit.
Stuff some kitchen scoures (spelling, you know what I mean) into stockings, tie them off a put one in each side of the catch can. Dont use steel wool as it can rust!
The stocking should catch the oil but let the air through, so now you have an oil/air seperator