Sub panel supplied by two different lines from main panel
Either it's 120V subpanel and he's plain paralleling... which is rather bad.

Or it's a 240V panel, and he's supplying hot/neutral/ground with one cable and the other hot with the other cable.  That's a different kind of bad.

Both are unacceptable.  You should replace both cables with a single /3 cable: 


4/3 Aluminum or 6/3 Copper if you want 60A in this subpanel
8/3 Copper if you want 40A
10/3 if you want 30A
12/3 if you want 20A
14/3 if you want 15A, but this is fairly pointless. 


And, both hot wires need to land on a 2-pole breaker which has common trip, meaning both legs trip together, and, move either handle, the other goes too.  



Note that a duplex/tandem/twin/double-stuff has independent handles and must not be used, as it will create a dangerous situation if you do.