If you use the multiple tax-file method, you'll probably have to allocate all of the Maryland tax withholding and estimated tax payments to the tax file used to file your federal tax return. That will probably mean zeroing the Maryland tax withholding in the other file so that you aren't credited for the same withholding on both the Form 502 and the Form 505. Forms 502 and 505 both require you to attach (included electronically when e-filing) Forms W-2 and 1099 that show tax withholding that you claim on the particular form.
