One Or Two.

Their reliance on each other cuts back on their need of Origin essence for resurrection. They can use their Origin essence to grow faster by investing it in making their concepts stronger. This helps them to take full advantage of the speed of their two minds working on comprehending the same law.

So not only do they return from the dead faster, they also comprehend laws faster and become stronger faster. These are all advantages that they will lose if they make two Supreme laws. 

Concepts don't work well with each other because they interact, and when they do, one of them must manifest above the other. One concept must become dominant while the other is repressed. This antagonistic reaction is suppressed in Geminis thanks to their separate Origin. But a Supreme Law will not be held back by an Origin.