How would one describe the relationship of next-to-adjacent?

In other words, say there are three or more objects, how would you describe the relationship between object one and object three?

For context clarity, I am referring to the lateral positioning relationship of teeth. I am discussing the relationship between tooth number 7 and tooth number 9. Number 8 in this situation would be the adjacent tooth. What would number 9 be?

I have found the terminology 'second adjacent' used in the way you are seeking.

It is used in a Wikipedia article discussing broadcasting channels.

Radio World also uses the terms 'second adjacent' and 'third adjacent' in respect of the same background - that of radio channels, meaning the next to adjacent and the next one after that.

Assuming that you don't just refer to tooth number 9 (since it has a designation anyway), I would use on the other side.

As in:

I know this incisor is healthy, but what about the premolar on the other side of the canine?

Or, in other words, tooth 8 is between teeth 7 and 9, and teeth 7 and 9 are on either side of tooth 8—or they surround it.