Forshayer: You wrote “The one point that I would like to state is that it seems like most of the earlier Poskim who give their opinion seem to be basing it in a case where only one person says each Kaddish. ” I am not sure what you mean by “earlier poskim” when the Gesher Hachayim was written in the 1900’s and he bases kadima for the amud based on the kadima for kaddish.
The Nitei Gavriel states this outright in Hilchos Aveilus Chelek Beis Perek Nun Hey Sif Yud Beis:
http://www.hebrewbooks.org/pdfpager.aspx?req=46540&st=&pgnum=404