I don't know the definition of the weight of t1. If it is A1/(A1+A2), then there are two methods.
Method1:
In Origin 8.6, you can specify the weight of t1 as a Derived Parameter.
Method2:
You can also specify the weight of t1 as a fitting parameter, i.e.
y = y0 + A*(p*exp(-(x-x0)/t1) + (1-p)*exp(-(x-x0)/t2))
where p is the weight of t1.
Generally the two methods should get the same result.
Sam
OriginLab Technical Services