Stacking yeast means when you add more than one sachet of yeast to a larger fermentation volume.
You can stack with Still Spirits Turbo Yeast, but you have to make sure that you scale up the water and sugar volumes as well. If you don't do this, there could be an imbalance of nutrients in the wash which can lead to stuck fermentation, blue coloured spirits and bad alcohol yield. If you scale up properly then there shouldn't be a problem.
The other important factor is temperature. You will need a method to control the temperatures as fermenting large volumes cause excessive heat to be generated. You must thus find a way to cool the fermenter so the temperature stays in the range declared on the yeast pack.
Due to the above, Turbo Heat is the best yeast to use for stacking as it is resistant to high temperatures.