pool3.scala

val cs1 = IO.contextShift(ec1)
val cs2 = IO.contextShift(ec2)

def run(name: String)(th: IO[_]): Unit = {
  println(s"-- $name --")
  th.unsafeRunSync()
  println()
}

run("Shift") {
  printThread *> IO.shift(ec1) *> printThread *> IO.shift(ec2) *> printThread
}

/*
Outputs:

-- Shift --
main
ec1-1-274064559
ec2-1-1945604815
*/