She was able to hold Joline, Tesline and Edessina, which leads me to believe that splitting flows doesn't really mean splitting the strength of an individual flow. Speculating here, but we know this is a skill feat not a strength feat. Perhaps the reason it's so difficult has to do with maintaining power in each weave rather than dexterity. We saw Egwene create 14-way split, while nearly depowered, yet her lights didn't get dimmer with each addition, seems to me that as power is released new power is drawn thus allowing a channelers splitting flows to power each flow at a near full strength level for each individual flow.

Rand holding Egwene and Elayne in that scene doesn't necessarily mean he was only using a fraction of his strength to weave each shield. Only that he had to maintain multiple weavings.