I think you can do it. It would be vital that you establish a different qualified retirement account that is not included in the original SEPP. Check out this website for good info and then back it by reading referenced IRS docs
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Edit - I have zero knowledge of 401(k)s, but I think you can make penalty free withdrawals if you leave your employer after you turn 55. Somebody will come along and correct me if I am wrong, but if that is the case I don't guess you would want to burden yourself with the potential complications of dealing with a SEPP. But you mentioned both IRAs and 401(k), are you planning on doing your SEPP from both?