Help with a validation issue..? Tying 2 validations together

Hi,

I am trying to validate a field upon the presence of a value of
another field -- the two are linked

for example my records have fields like this:

    t.decimal "admin_headcount", :precision =>
10, :scale => 2, :default => 0.0
    t.decimal "admin_comp", :precision =>
10, :scale => 2, :default => 0.0

My forms have fields for entering of both values, however, the form
should post an error if the user tries to submit the form if there is
a non-zero value for headcount but the compensation amount is zero,
and vice versa.

Is there a way to tie
     validates_numericality_of :admin_comp
     validates_numericality_of :admin_headcount

to each other..?
In essence: validates_numericality_of :admin_comp,
unless :admin_headcount <= 0

any simple and obvious way to accomplish this..?

regards,
  DHMS

validates_numericality_of :admin_comp, :unless => Proc.new{ |r|
r.admin_headcount <= 0 }