John Gill Commentary 2 Samuel 15:36

"Behold, they have there with them their two sons, Ahimaaz, Zadok`s son, and Jonathan, Abiathar`s son; and by them ye shall send unto me everything that ye shall hear." — 2 Samuel 15:36 (ASV)

Behold, [they have] there with them two sons, Ahimaaz, Zadok's
[son], and Jonathan, Abiathar's [son]
As in (2 Samuel 15:27) ; not that they were in the city with them, but they were near it, (2 Samuel 17:17) ; with whom they had a communication:

and by whom you shall send to me everything that you can hear ;
that is, by the sons of the priests; he telling the priests how things were at court, and they sending their sons with messages to David; which was a good scheme to get intelligence, and easy to be put into execution.