is it possible for a person (specifically an intending u.s. immigrant)
to purchase more than one medical insurance policy, e.g., from
different companies (to maximize coverage)?

Of course you can, generally speaking;

However you can not claim the same amount twice, for instance if you
go to a Dc. and the bill comes out to be $ 1,000 the 1st insurance
company will have a deductible of say $ 600 and cover the remaining
$400. You can not claim any part of this bill on the second insurance.

If your bill comes out to be $ 10,000 but your 1st insurance has a
maximun payout of $3,000 per year you can claim the remaining $ 7,000
on the second insurance up to your maximun, after you meet their
minimum deductible and ussually you can not claim the original $ 600
again.

That said, it is often better to get greater coverage on one insurance
policy than two mediocre ones. There is easier, less paperwork and
because it comes 'bundled' you get more for your money with less
deductibles.