I use Z21 on an ipad (which is full up now) however when I want to run double headed - consist is a bit modern for 1962 - I have a twin page open (not possible on an iphone etc) and then simply work the controls together, even absolutely identical models vary a bit so it is a bit of adjustment needed to get them running nicely and indeed I do this to run locos in, so all this messing about with codes I find unnecessary as a result. I do have 128 speed steps without bother. Not sure why you have the reported issues but has the device got enough puff? I found that the android I had was full at 100 locos but still worked OK to then? is yours a black box Z21? as mine is.