Help with a 2-argument markup command definition

[Robert Schmaus]

Hi everyone,

I'm trying to define a 2-argument markup command. the definition below 
doesn't work, I get all sorts of errors when I call either
\jcStack{"x"}{"y"}
or
\jcStack "x" "y"
or
\jcStack #"x" #"y"



#(define-markup-command (jcStack layout props jcHi jcLo) (markup? markup?)
    (interpret-markup layout props
      (markup #:smaller jcHi #:larger jcLo )
    )
  )


does anyone know what's my mistake here? and which one of the function 
calls above would be the correct ones?
(Remark: I already defined commands with 1 argument, and those work fine 
...)

Thanks!
Best,
Robert