I would not taper down cannabis until you are taking Lamictal at 100 mg at least and for a while, but if your dr suggested cross tapering and you are feeling fine, continue what you have been doing. In general, there are three ways to switch from drug A to drug B -

Discontinue A and start B
Continue A while you ramp up B and only then stop A
Taper A down and B up

Which option to choose depends on many factors, including interactions between A and B. Cannabis and Lamictal do not produce untoward interactions. I guess your pdoc simply opted for middle ground.