If he has side effects from Humira then I would separate them if he's just starting the MTX. If would confound your recognition of MTX side effects to do them together. Plus he may have side effects from both and be really sick for a day or two so if you're going to do them together for the first time I'd do them on a Friday or Saturday to give him time to recover. I would also premed with antihistamine.
We did MTX oral on Friday's because he would feel really tired the next day and it was the weekend. We did Humira EO Wednesday I think (it's been a while).