Pruning during flowering is not advised as the plant will be forced to divert its energy from bud production into branch and leaf production. This results in a slower rate of bud growth. For optimal growth finish your pruning well before flowering. I usually start topping around 7-8 nodes. But i like them bushy. If you top it now the two top colas will simply share the same volume of bud that a single cola would have produced the same.