ok right branch this time (#19110)

code/modules/assembly/assembly.dm

@@ -105,6 +105,7 @@
 	holder = new /obj/item/assembly_holder(get_turf(src))
 	if(holder.attach(A, src, user))
 		to_chat(user, "<span class='notice'>You attach [A] to [src]!</span>")
+		user.put_in_active_hand(holder)
 		return TRUE
 	return FALSE
 

code/modules/assembly/holder.dm

@@ -179,12 +179,15 @@
 		var/turf/T = get_turf(src)
 		if(!T)
 			return FALSE
+		user.unEquip(src, TRUE, TRUE)
 		if(a_left)
 			a_left.holder = null
 			a_left.forceMove(T)
-		if(a_right)
+			user.put_in_active_hand(a_left)
+		if(a_right) // Right object is the secondary item, hence put in inactive hand
 			a_right.holder = null
 			a_right.forceMove(T)
+			user.put_in_inactive_hand(a_right)
 		qdel(src)