To prevent underfilling or overfilling (and flashing) of either material, the shot-to-shot accuracy of the machine is obviously a critical factor.

Shot variability of less than 0.3% to 0.5% is recommended. A machine with closed-loop injection-speed control is the best choice.

Next, pick a mold maker with experience in multi-material parts. You can save a lot of money if you have the mold designed well from the start. For example, it can be helpful to supplement the thermal/chemical bonding between two materials with a mechanical joint achieved by using undercuts or similar designs.

Make sure multi-cavity molds are well balanced. Hot-runner manifolds must be balanced too, and the number and size of drops must be sufficient for low-pressure filling.
Mold temperature is another important factor. Accurate control of the temperature is mandatory when running molds with core lifters for the second shot. Incorrect mold temperature can cause a lifter to wedge or jam, because of differential thermal expansion of the steel or steel/brass combination.

Operators must be well trained for successful multi-material molding. Wrong machine settings are often the culprits when parts don’t turn out right. Because of its complexities, only people who understand the process should be allowed to attempt corrections if something goes wrong.
Achieving a good bond between materials is often dependent on the temperature of the first material when the second is injected.