There is no supported way.
There are ways that you could run your own code that would find the axes that is being used for the curve fitting toolbox and mark the axes as the equivalent of "hold on". and then you could choose new data sources and functions and run those through. However, by default cftool runs (and plots) after every change of everything, instead of just when you ask it to, so the output is likely to get crowded.
I would suggest to you that it would be more productive to do the fits and save the fit objects when they are ready, and then run code to plot the various objects in other windows.
