When performing the litanies in congregation, form only a single voice because this represents the Oneness of ALLAH. If you do not form only a single voice, ALLAH does not recognize Himself in the zikr where each person forms several separate groups, and no longer a single group, and in this case they lose their strength because ALLAH is with the group that, out of several people, becomes only one. It is related to the verse which says:
"And do not dispute and (thus) lose courage and (then) your strength would go away" (Sourat Al Anfal/the spoils of war, verse 46).
“Do not dispute” means not to divide, because the oneness is broken and then ALLAH does not recognize Himself in this group and does not support it anymore, so they lose their strength, and the same goes for the zikr in congregation out loud.