  Parallel computing and steadily increasing computation speed have led to a
new tool for analyzing multiple datasets and datatypes: fitting several
datasets simultaneously. With this technique, physically connected parameters
of individual data can be treated as a single parameter by implementing this
connection into the fit directly. We discuss the terminology, implementation,
and possible issues of simultaneous fits based on the X-ray data analysis tool
Interactive Spectral Interpretation System (ISIS). While all data modeling
tools in X-ray astronomy allow in principle fitting data from multiple data
sets individually, the syntax used in these tools is not often well suited for
this task. Applying simultaneous fits to the transient X-ray binary GRO
J1008-57, we find that the spectral shape is only dependent on X-ray flux. We
determine time independent parameters such as, e.g., the folding energy E_fold,
with unprecedented precision.
