Levelling heads for dual extruder
Make sure that the bed is level. As the saying goes, a level bed is next to godliness or something like that. Pay extra attention to the direction the nozzles are offset by (if one nozzle is offset on the X-axis, pay extra attention to the bed leveling along the X-axis). This can be done with just one nozzle and a business card or piece of paper.
Use a bubble level to get the nozzles about right. Move your z-axis up a bit and put a bubble level against the nozzles. Adjust as necessary so it's exactly level. The nozzles should be level enough that the bubble stays in the middle.
Fine-tune it with a business card. When you home the z- axis, you should be able to just fit a business card under both nozzles with a moderate amount of resistance. Don't force the card. If sliding the business card under produces a different amount of resistance for one nozzle than for another, adjust the nozzle a tiny amount. You can also use an index card or playing card.
Once it passes the card test, try a test print. If it doesn't work, make sure your bed is level, your nozzle offset is correct in the slicing software, and try calibrating with an index card again. If the nozzles become way off, try the bubble level again.


As for physically adjusting the level, another answer suggests shims made from aluminum foil, which work well. Personally, my extruder was off-level by almost exactly 1mm, so a pair of washers worked nicely for that.

Happy printing!

Leveling with a bubble: