documentclass[twoside]{article}

usepackage{xparse}
makeatletter


NewDocumentCommand sidenote { +m }
{
  marginpar{#1}
}
%letsidenotemarginpar


newbox@mpbox
globalsetbox@mpboxvbox{}
defsavedpos#1#2#3#4{%
begingroup
let@positionsrelax
expandafterxdefcsname sp@#1-#2endcsname{%
expandafterifxcsname sp@#1-#2endcsnamerelax
else
csname sp@#1-#2endcsname
fi
  @positions{#3}{#4}}%
endgroup}

defmarginpar#1{%
  saveposition{mpar}%
  globalsetbox@mpboxvbox{unvbox@mpboxhbox{%
  hbox{parbox{marginparwidth}{@marginparreset#1}}%
  hbox{parbox{marginparwidth}{@marginparreset#1}}%
}break}}

defsaveposition#1{%
pdfsaveposwrite@auxout{%
  noexpandsavedpos
    {#1}{thec@page}{thepdflastxpos}{thepdflastypos}}}

def@oddfoot{%
  hssthepagehssrlap{hskipmarginparsepmcolumn}}
def@evenfoot{%
  llap{mcolumnhskipmarginparsep}hssthepagehsssaveposition{foot}}

defmcolumn{%
saveposition{foot}%
expandafterifxcsname sp@foot-thec@pageendcsnamerelax
else
let@positionsorigin@positions
csname sp@foot-thec@pageendcsname
smash{raisefootskipvbox to textheight{hsizemarginparwidth
hrule@heightz@
let@positionsmp@positions
csname sp@mpar-thec@pageendcsname
vskipz@@plustextheight% not here
hrule@heightz@}}%
fi}

deforigin@positions#1#2{%
@tempdimaz@
dimen@textheight
advancedimen@headsep
advancedimen@ #2sp
}
defmp@positions#1#2{%
setboxtw@=vsplit@mpbox to maxdimen
setboxtw@vbox{%
unvboxtw@
setboxtw@lastbox
setboxtw@hbox{%
unhboxtw@
ifoddc@page
globalsetbox1lastbox
fi
globalsetbox1lastbox
}}%
@tempdimbdimen@
advance@tempdimb-#2sp
ifdim@tempdimb<2p@
@tempdimb2p@
fi
vskip@tempdimb@minus@tempdimb% not here either apparently
advancedimen@-@tempdimb
advancedimen@-ht@ne
advancedimen@-dp@ne
hrule@heightz@
box@ne
hrule@heightz@
}

makeatother

% just some filler text, equations, etc.
defsomeequation{%
begin{equation}
left[frac{hat p^2}{2m}+V(r)right]psi(r)=Epsi(r).
end{equation}
}
defelementarytext{%
One two three four, united states marine core.
}
defsometext{%
elementarytextelementarytextelementarytextelementarytextpar
elementarytextelementarytextelementarytextelementarytextpar
elementarytextelementarytextelementarytextelementarytextpar
}

begin{document}

sometext

elementarytext
someequation
elementarytext
someequation
elementarytext
someequation
elementarytext
someequation
elementarytext
someequation
elementarytext
someequation
elementarytextsidenote{Here is a side note with some text.}
someequation
elementarytextsidenote{Here is a side note with some text.}

end{document}