Investors will have problems if the gap is too large. They'll wonder what discussions aren't being had with the 20% cofounder, if that person is motivated to be part of the company for the long term, etc.
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.
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.