Unless you've done this many times before, its not really that big a deal to do an even split. Assuming success, you'll likely end up with less than half of your original equity, so what is a 20% gap now (huge!) might be 5% or less when you exit (ie no big deal).
Of course, go with a vesting schedule and cliff. And to make up for them coming on later, perhaps you start with 40 / 40 / 20 but have a plan (that you share with them) to grant them additional equity each year until it's roughly even, so as they prove their worth and commitment you make them an even cofounder.
Maybe in some corner case where it isn't a waste of energy? That's why it is worth at least considering Joel Spolsky's advice:
https://gist.github.com/isaacsanders/1653078
By the way, 40-40-20 is no better from a control perspective than even thirds. Two can sell the other out. And control is a bigger issue than percentages in the sort of pissing contests that matter. Even worse, the person with 20% has more to gain by cutting one of the 40%'s out...as do each of the 40%'s.
When would it make sense to go 40%-40%-20%? And how would it look like when we bring in funding from outside investors?