1)
Why did he say "Lachem" (plural)?
1.
Radak #1: This is an esteemed expression (the royal 'we').
2.
Radak #2: You and the Chachamim with you.
3.
Malbim: Achitofel must draw Chushai and counsel also with him. Avshalom preferred Chushai's approach (to slowly force David to cede to him) to Achitofel's (to destroy David). He asked Achitofel first, for Achitofel's counsel was like asking Hashem (verse 23). He wanted both to agree on a counsel. 1
1
Malbim (17:1): Achitofel did not do so, for he knew that Chushai would not agree to kill David.
