Can I have two insurance policies, employer and marketplace?

So, I’m starting a new job that’s contracted through an agency (W-2’d). The insurance policies offered by the agency aren’t great. I’m looking to see if I can sign up for both the basic employer plan AND a marketplace plan during my special enrollment period. The employer plan is great for doctor visits and drugs, but only covers that (no testing, imaging, hospital, mental health, etc). (They do have a second, high deductible plan that covers these things.) I’m looking to add on a marketplace plan on top of the basic employer plan that would cover testing, imaging, hospitalization, urgent care, etc. Can I do this? Would I need to decline employer coverage to enroll in marketplace? If it was open enrollment, would it be any different? Would I be able to enroll in one plan now, and add on the other later during next open enrollment?

I’m not eligible for ‘savings through the marketplace’, I assume this is already taken into account when I put in my income on the marketplace healthcare website, and est policy pricing reflects this. So, losing any savings by having two policies/the option for employer policy isn’t an issue. I’ve worked out that if I do this, I can get all the benefits I need and save about $100+ a month on premiums.

Also, if I can proceed with this – if I see a doctor with insurance 1 and they prescribe an MRI, can I get the MRI with insurance 2? Insurance 1 wouldn’t cover an MRI, so would it be passed on to secondary insurance (insurance 2) after insurance 1 denied it, and then [likely] covered? I understand no one can say definitively.