[Q] Is a "concurrent" filing allowed so that I can file I-526 immigrant petition and I-485 together at the same time?
No, although "concurrent" filing is allowed for some Employment-based categories, I-526 immigrant petition and I-485 adjustment cannot be filed together at the same time in the United States. USCIS does not like "concurrent" filing too much, so we do not see USCIS allowing concurrent filing in near future, if ever.
"Concurrent" filing is helpful when the applicant will have a hard time maintaining status while an immigrant petition is pending. This means it will be very difficult for an applicant who entered the United States to file I-526 petition and then be able to adjust in the United States.