I don't believe these two species are compatible. A. rubrum is in the Rubra section while A. platanoides is in the Platanoidea section.
If I'm not mistaken, it is rare for species in different sections to be compatible (although there are a couple of species that are compatible with several, pentaphyllum and pseudoplatanus I think). Species in the same section are sometimes grafted together, as they are more likely compatable.
However, every tree within it's species has slightly different DNA, and the two trees you are referring to could have just enough genetic resemblance to mend.

To encourage inosculation, should I plant saplings far-enough apart so that the trunks, first, grow to 1/3 mature width before they expand to the point of making contact with eachother? I figure that if they start of only 2 or 4" inches apart, they will not develop a healthy core; on the other hand, maybe this is this best way?