[Chris Saper]

Sophie,

If I were in your position, I would stress the heirloom nature of the kids' portraits, and that it would be a much better solution to paint a pair of separate portraits, designed to hang as a pair, but still separatable for the children to take to their own homes when they are grown.

At a glance, I am not seeing combinations where the light on both subjects is similar enough to look as if they were actually sitting together. You would need several photos with both children to know how to properly size their heads.

Lastly, the only reason I can see to paint "together" instead of "separate" is to convey an interaction or relationship, and none of these combinations show the kids relating to one another. And perhaps they don't relate to each other...in which case you can tell the parent that they are so uniquely individual in their personalities that each should have the chance to shine.