After a small search on the internet using Google, I found the following text:
You may notice that there is no Lemmings Bomb option available on the
screen. If you have come to a draw or would like to restart the level,
both players must move their cursors to the green and blue bars in the
middle of the screen just above the Lemming icons. When this task is
performed, the colored bars will turn into a Lemming Bomb icon. Both
players must press their “A Button” twice simultaneously to “nuke”
every Lemming on the screen and thus, end the level.
