The danger is in some twins is that if you have full tanks and crossfeed (accidentally) only one engine, the fuel return is pumped into the opposite (full) tank and hence overboard. Combined with two thirsty engines feeding off one tank at say 30+ gals per hour (total) and you can run it dry in double quick time and end up with double engine failure but a full fuel tank. If you spot it of course you can crossfeed back and save the day but you have pumped half your fuel o/b and now your range is significantly impacted.