Never heard of leo that doesnt eat both. Once again this is one of those times where the leo has a mind of its own.
Heres what I would do, either feed them 1 or the other. Not both even tho that seems logic. Mealworms and Crickets have the same as far as energy and nutrients. THATS ACCORDING TO REPTILEFOOD.COM
(AND JUST AS A NOTE, I HAVE NEVER HAD A LEO THAT DOESNT EAT BOTH SO THIS IS JUST A GUESS)
As I was saying, if you want him to eat mealworms, don't give him that choice of crickets. If you do, then hes going to take what looks better without trying the other.(kind of like my little sister). Hes going to get Hungry, and if he sees the mealworms are his only option, someones going to find out that mealworms arent as bad they may look.
And if your leo ends up being stubbord, you can do either 2 things.
1.)Serve both Meals and Crickets
2.)Strictly Crickets knowing that everyone will eat them.
And just as a next note, if crickets do in fact have certain vitamins and stuff in it that mealworms dont, by the vitamins seperately. Rep Cal, or whatever. =)