I have 2 Hitec digitals on a Y-harness to drive split elevators on a Seagull Harrier .90.
I use a Hitec Supreme 8 receiver and an 1800mAh NiMh 5 cell battery (6V). I have not seen any problems in this configuration. Not sure what's up with your setup.. Just thought I'd let you know that I'm running a similar setup successfully. Oh the Y-harness is a 'buffered' harness (i.e. uses some battery power to reconstruct the servo signal to each servo independently)... Have you tried to buffered Y-harness??