If you parallel two feeds of different length (hence, different impedances) the one with lower impedence will carry more of the current demanded by the load......hence, current imbalance. Think of two branches in parallel with different resistences, they have differenet currents. Taking into consideration, connections, circuit lengths (though a house), etc, etc. they current imbalance can be such that even though the load is not drawing more than the two feeders can supply (30A = 15A + 15A), one feeder may be overloaded (30A = 12A + 18A).