I've spent a couple of days to understand how to boost stepper motor shields.
Some higher powered driver chips do exist but the absolute limit seems to be around two 2.5A full bridge drivers - 4A peak - to power one single stepper.
Considering the microstepping control and overcurrent protection of those chips which is regulated over Isense resistors in the ground return path of the low side of each bridge, it is no simple task to boost the maximum current of the system by just driving another heavier external bridge.
It can be done though, theoretically, by taking into account an adapted current limit detection circuit on the external bridge and a suitable driving mechanism using the bridge outputs of the controller chip.
Also, in order to respect the regulation loop for the internal PWM and DAC comparators that organize the microstepping, you MUST take into account the extra delays added to the loop by the external power bridge. Could be tricky.
In any case, you will have to lower the maximum switching frequency of the controller.
If that's not enough to take care off, you will be in big trouble still if the controller driver starts to use these mixed modes in the PWM/DAC controller where it drives only the high side or the low side of the bridge. You are not able to easily detect and implement these modes externally because on most controllers these pins are not available.
Now, with all this in mind, you will definitely blow a couple of dozens of chips and bridges before you will be in the neighbourhood of a good stable and reproducable solution. In any case you most probably will need to disable mixed driving modes.
It might be wiser to leave this effort to the new chip designers working for very rich companies and buy a 75$ industrial controller that can handle 6A steppers with many extra's on top of that.
The company that builds a single chip controller in SMT that can handle 6 or 8 amps with extra features will be in a good shape. But do they want to do that? It will surely decimate a part of the existing multi billion dollar turnover industrial control market...