discussed Cited "see" Southern Electrical Health Fund v. Heritage Mutual Insurance See Int'l Longshoremen's Ass'n, Local Union No.1937 v. Norfolk S. Corp., 927 F.2d 900 , 902-03 (6th Cir.) (noting that whether two companies are acting as a joint employer is a factual issue and, ultimately, concluding that the district court’s determination was clearly erroneous), ce rt. denied, 502 U.S. 804 , 112 S.Ct. 46 , 116 L.Ed.2d 24 (1991); see also Boire v. Greyhound Corp., 376 U.S. 473, 481 , 84 S.Ct. 894 , 11 L.Ed.2d 849 (1964) (question *506 of status as joint employer is “essentially a factual issue”).