The author wants to affirm that David had been back in Ziklag for two whole days before he received news of Saul’s defeat on the third day after his return. Presumably these days were used to rest and begin the restoration of the homes which had been burned (1 Samuel 30:1–3) as well as sorting through the plunder (1 Samuel 30:26–31).1
1 After the death of Saul, when David had returned from striking down the Amalekites, David remained two days in Ziklag.