[R] Multi-dimensional non-linear fitting - advice on best method?

[Julian Gilbey]

Hello!

I have a set of data of the form (x, y1, y2) where x is the
independent variable and (y1, y2) is the response pair.  The model is
some messy non-linear function:

  (y1, y2) = f(x; param1, param2, ..., paramk) + (y1error, y2error)

where the parameters param1, ..., paramk are to be estimated, and I'll
assume the errors to be normal for sake of simplicity.

If there were only one response per input, I would use the nls()
function, but what can I do in this case?

Many thanks,

   Julian