Mich. Comp. Laws § 418.372

Sec. 372. (1) If an employee was engaged in more than 1 employment at the time of a personal injury or a personal injury resulting in death, the employer in whose employment the injury or injury resulting in death occurred is liable for all the injured employee's medical, rehabilitation, and burial benefits. Weekly benefits shall be apportioned as follows: (a) If the employment which caused the personal injury or death provided more than 80% of the injured employee's average weekly wages at the time of the personal injury or death, the insurer or self-insurer is liable for all of the weekly benefits. (b) If the employment which caused the personal injury or death provided 80% or less of the employee's average weekly wage at the time of the personal injury or death, the insurer or self-insurer is liable for that portion of the employee's weekly benefits as bears the same ratio to his or her total weekly benefits as the average weekly wage from the employment which caused the personal injury or death bears to his or her total weekly wages. The second injury fund is separately but dependently liable for the remainder of the weekly benefits. The insurer or self-insurer has the obligation to pay the employee or the employee's dependents at the full rate of compensation. The second injury fund shall reimburse the insurer or self-insurer quarterly for the second injury fund's portion of the benefits due the employee or the employee's dependents. (2) For purposes of apportionment under this section, only wages that were reported to the internal revenue service shall be considered, and the reports of wages to the internal revenue service are conclusive for the purpose of apportionment under this section. (3) This section does not apply to individuals entitled to benefits under section 161(1)(d), (e), (f), (g), (h), (i), (j), and (o).