Two Stepper Motors from one Controller

I have built a Prusa Mendel and have two Nema 17's on the top to run the Z axis.

Each motor has 4 wires which I have connected together. Each matching colored wires are connected together, and are then connected to the 4 wires from the Controller. What happens is that the motors do not run in unison as I expected.
Sometimes they do, sometimes one motor will run very fast while the other won't run at all. I know very little about electronics so it wouldn't be a big surprise if I have it all wrong. Can someone please suggest how these should be wired? See attached rough drawing of how I have these two motors wired. Thanks for any help you can give.

Can you confirm both motors are same make and model? If not there is a risk they will never work well together.

Do you get identical multimeter readings when you test the identical wire pairs from both motors?

I have found it a good idea to solder wire connections and then use block connectors on the soldered joint - just to ensure good contact on all the wires.

Hi and thanks for the quick response. Yes motors are same brand, same everything. Now, I am sorry, but I don't know what I should be looking for when using a multimeter. I have one, but what should I be looking for?

It is probably just the case that one screw is a bit harder to turn and you are trying to go quite fast. The motor with the stiffer screw stalls while the other runs.

Make sure the axis is not binding. Try moving a bit slower or increasing the current a bit. If you find you want more torque you can wire them in series instead of parallel but the max speed will then be lower,

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Also check, by turning the axe with your fingers, that the force needed is about the same on each side.

If it is not you might have misaligned one of of the rods, which makes one motor failing earlier when increasing speed as it needs more torque to turn.

I have found that the parallell wiring of steppers is a bit unstable (at least with the pololus and the sy42sth47-1684), the z motor bearing the most weight (the one over the x motor) tends to lose steps & stall.
Parallell wiring would work decently if both motors had the same load.
I solved this by wiring the motors in series. The series wiring is briefly described at the wiki page as an alternative wiring method. Take your time to draw your coils and color-code. Worked well for me.