One of my friends, training partners, and occasional training clients and I were talking last night.
One thing that came up was mixing two approaches.
It really applies widely to diet and fitness.
I can't count how many times I've been told someone is doing Program X plus Program Y.
"I'm do P90X! And Couch to 5k!"
"I'm doing Starting Strength, plus my marathon prep."
"I'm doing 5/3/1, with Crossfit on my off days."
"I'm training with you twice a week, and I'll do two or three randomly chosen classes a week at my local fitness club!"
And back to diet:
"I'm cutting out meat and I'm cutting out gluten and starchy carbs."
"I'm counting calories and I'm trying to eat until 80% full."
Or a combo:
"I'm doing a mass gain program and cutting my diet to lose weight."
In a lot of cases, the goals don't overlap. In others, they do (or at least don't conflict.) Some might work fantastically together . . . if you were already doing one, and started to add on the other to your existing routine little by little.
The temptation to double is is great - you want to double your progress! Why do one diet and lose a few pounds of fat when you can do two and lose twice as many? Why not add the core elements of one program to another program with competing core elements and let them fight it out while you reap the benefits?
You get two big problems.
Overload - You're trying to do two things before you've mastered one. If you could more easily pick up habits two at a time than one at a time, people would adapt to new things in pairs all the time. It's twice as much information to process. It's twice as much calculation to do. It's at least twice as many chances to have more things to do than time to do them.
Conflicting Demands - Generally each eating approach or training approach assumes that is the only approach you are using. Or at least, assumes you are meshing it with something that doesn't interfere with your main program. Programs might demand you go heavy on Tuesday and light on Wednesday, yet your other program has you ramping up intensity regularly in weekly waves. Now you're going hard on Tuesday and hard on Wednesday and hard again on the days after. Something is going to give out, and it's either your compliance or your body.
If you really want to do both programs, or both diets, consider doing them sequentially. Do one for a while (a cycle, two months, six months, whatever) and see how the results are. Go do the other and compare. Doubling up doesn't multiply your progress, it probably halves it. You are better off committing 100% to a program that is less than all of what you want than committing 50/50 to two programs that are everything you want.
Or sit down, by yourself or with a trainer, and find a way to make them play well together. Don't be surprised if two approaches developed in isolation from each other don't work well together. You can't always make them play well together.
Just keep this guideline in mind: doing two approaches for one solution is halving your progress.