documentclass{article}
usepackage[utf8]{inputenc}
usepackage{pgf}
begin{document}
pgfkeys{
    /keys/.is family,
    /keys/.cd,
    test 1/.store in=testone,
    test 1=unset,
    test 2/.store in=testtwo,
    test 2=unset,
}

pgfkeys{/store/.code={pgfkeys{/keys/.cd,#1}}}
defmykey{test 1=ONE,test 2=TWO}
pgfkeys{/store=mykey}
t1 : testone,
t2 : testtwo.
end{document}

t1 : ONE, t2 : TWO.

t1 : unset, t2 : unset

Package pgfkeys Error: I do not know the key '/keys/test 1=ONE,test 2=TWO'
and I am going to ignore it. Perhaps you misspelled it.

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{pgf}
pgfkeys{
    /keys/.is family,
    /keys,
    test 1/.store in=testone,
    test 1/.initial=,
    test 2/.store in=testtwo,
    test 2/.initial=,
}
newcommandsetkeys[1]{pgfkeys{/keys, #1}}

begin{document}
  setkeys{test 1=ONE,test 2=TWO}
  t1 : testone,
  t2 : testtwo.
end{document}