Michigan has no UM/UIM statute, so your policy decides, but the no-fault act sets the tort claim behind it
Michigan repealed its uninsured motorist statute, MCL 500.3010, in 1972. The tort claim your coverage rests on is governed by MCL 500.3135, liability limits by 500.3009, the first-party medical choices by 500.3107c and the deadline by MCL 600.5805.
Read how stacked and single-policy coverage differ in Michigan, check what the statutes say and leave to your policy, or see how UM and UIM work under Michigan no-fault.