Linking an Arduino Mega with RAMPS and an Arduino Uno with CNC Shield to control more stepper motors using Marlin firmware
so points to that case:


how are you going to sync master/slave?
adding more steppers to master (even only for pushing control to slave) will limit speed in the whole system (this is mainly one of the reasons why we shall go to 32-bit platforms to print faster).


As marlin will give you a good grasp to convert g-code into steppers movement, then if you are willing to double or triple steppers number per axis, you can grab dir and step signal and forward it to other steppers drivers without the mess of syncing movement in time and board. If you are going to add a special tooling - then you shall add your mods to firmware as well.