Efficiency drop when joining and splitting teams
Has there has been done any research on why you should or should not
  join or merge teams in short time periods?


One big difference in how the troops were maintained in the second world war was that for the allied forces when a person died/got seriously injured this person was always replaced with another one (under the assumption "you always want the troops to be fully staffed"), whereas the Germans did not replace troop members and the troop kept its original staffing until it became too small when it was just broken up (motivated by group dynamics as you are asking about).

It seems at first a bit counter intuitive, but the Germans performed better due to their strategy than just continuously replace personnel. I am sure there exists a lot of research into this.