simultaneous fitting

Hey all,

I have been mulling this over for quite a while and I can't seem to come up 
with an adequate answer. What I'm trying to do is fit both the plus and minus 
orders of a grating together. Now if I want to fit them simultaneously. I just 
load both in and use xnotice to tell which datasets and what wavelengths I 
want to fit. That seem relatively straightforward. What I want to know is the 
fundamental difference between fitting in this manner, and first combining the 
two and then trying to fit. I understand essentially what it means to combine 
datasets, but when I try to fit them, I'm not sure what I should put in. For 
example

g=combine_datasets(heg+1,heg-1);
hegcomb=get_combined(g,&get_data_flux);

now if I say xnotice(hegcomb, 1.7, 2.3);

xnotice doesn't understand this structure type. If I tell it:

xnotice(1, 1.7,2.3); 

It says ranges for 1 and 2 do not match. If I put in 

xnotice(1, 1.7,2.3);
xnotice(2, 1.7,2.3);

It doesn't balk, but the fit is pretty lousy versus just looking at one 
spectra at a time. Plus, the fact that it needs ranges for both the + and - 
order leads me to believe that it isn't fitting the combined spectra at all.  
Any light you can shed on this would be greatly appreciated.

Thanks,
Bert

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