Layering different types of heat transfer materials can certainly be done and will take your designs to the next level. However, make sure you check that the material you are wanting to use can, in fact, be layered. Most websites where you buy your product will say whether you can or cannot layer.
Tips for Layering
Stay within the brand! Don’t mix different brands of heat transfer materials. Chances are they either won’t work together at all or last for long.
Don’t do too many layers. Your design can get very thick on your surface if too many layers are stacked. Try doing an inlay or knockout design to avoid this problem.
When pressing your initial layer(s) make sure you are only pressing for a few seconds. This is long enough to tack down the material but will help with your initial layer(s) not shrinking due to the heat and thus your next layers not lining up.