Stacking is the same as doubling I think.
"Stacking" is the use of OPM to trade at a higher pip value.
I double because it makes my life easier; any size can be doubled and anything doubled can be halved.
You could place a trade for 5 units and add 4 units to reduce your space to 55.5% or
you could place a trade for 6 units and add 3 units to reduce your space to 66.6% or
anything that you want to do really.