Acts 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so.
12 Therefore many of them believed; also of honourable women which were Greeks, and of men, not a few.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
17 That the man of God may be perfect, throughly furnished unto all good works.
Neither verse claims that all Christian truth is in the Bible.
Acts 17:11 favorably compares the Bereans to the Thessalonians. The Thessalonians rioted when Paul preached to them, the Bereans compared his preaching with Scripture. Good for them.
2 Tm 3:16, in context, talks about the Scriptures which Timothy has known "since his infancy". That can't mean the New Testament, which he certainly hadn't known since his infancy (it hadn't been written), but must mean the Hebrew Scriptures. If 2 Tm 3:16 teaches the formal sufficiency of Scripture, then it teaches the formal sufficiency of the Old Testament alone.
But it doesn't. It says that the scriptures are profitable (certainly) and necessary for the man of God to be "perfect, thoroughly furnished unto all good works". "Necessary" is not the same thing as "formally sufficient; completely sufficient in and of themselves".