At the beginning of each end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)
When Wound Reflection’s ability resolves, it checks how much life each opponent lost over the course of the turn, then it causes that opponent to lose that much life. It doesn’t matter how the opponent lost life or who caused it. It also doesn’t matter if Wound Reflection wasn’t on the battlefield at the time some or all of the life was lost.
Wound Reflection’s ability checks only whether life was lost. It doesn’t care whether life was also gained. For example, if an opponent lost 4 life and gained 6 life during the turn, that player will have a higher life total than they started the turn with — but Wound Reflection’s ability will still cause that player to lose 4 life.
Wound Reflection’s ability triggers at the end of every turn. It doesn’t have to be your turn, and it doesn’t have to be the turn of an opponent that lost life.
If Wound Reflection’s ability resolves, then an opponent loses life later in the same turn, that life loss is never counted by Wound Reflection.
Multiple Wound Reflections are cumulative. They’ll both trigger at the same time. One will resolve first. When the second one resolves, it will see the same life loss that the first Wound Reflection saw, as well as the life loss caused by the first Wound Reflection. For example, if an opponent lost 3 life during the turn, the first Wound Reflection will cause that opponent to lose 3 more life, then the second Wound Reflection will cause that opponent to lose 6 life.