How sound is the narration of Saeed al-Maqbari and Abu Ishaq al-Sabi'i in the Two Sahihs, and did they narrate before or after their اختلاط (confusion/mixing of narrations)?

Abu Ishaq Al-Subai'i and Sa'id Al-Maqburi are trustworthy. What has been said about their "ikhtilat" (confusion/mixing up) has not been proven; rather, they experienced a weakness in memorization due to old age in their later years. Both Al-Bukhari and Muslim narrated from them in their Sahih collections, either because they considered their "ikhtilat" to be unproven, or because they meticulously selected from their narrations what was confirmed to be free from error and mistake.