Mary,
The problem with two's is that we want them to be three's, or one's.
In the first example below Bart Lindstrom uses the fence to suggest a third element. But in the next example, Sargent's "Mrs. Warren and Daughter," he brings the subjects so close that compositionally they almost show as one. Nice for mother and daughter but maybe not for father and son.
The next example is a little more figurative but shows an interesting interaction.
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Mike McCarty
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