Add clarification to security council page

docs/base-chain/security/security-council.mdx

@@ -37,22 +37,28 @@ Chain contract upgrades must contain:
 - a ≥75% quorum
 - a quorum-blocking group outside the main rollup operator (Base)
 
-In addition to Coinbase, the Security Council adds 11 independent entities and individuals from
-geographically diverse regions. To satisfy the ≥75% quorum, 9 out of the 12
-entities (the 11 entities in the Security Council and Coinbase) are
-required to approve Base upgrades. Base upgrades cannot take effect until a
-quorum of these entities sign and approve the upgrade. This composition
-satisfies all requirements.
+These upgrades are governed by a 2-of-2 multisig. Both parties must
+independently approve an upgrade before it can take effect:
 
-**Member Selection criteria**
+- **CB Signers** — a 3-of-6 multisig operated by Coinbase. For the purposes of
+  the overall quorum, the CB Signers count as a single signer.
+- **Security Council** — an 8-of-11 multisig composed of 11 independent entities
+  and individuals from geographically diverse regions.
+
+Counting the CB Signers as a single signer alongside the 11 Security Council
+members gives 12 participants in total, and any upgrade requires the approval of
+9 of them (the CB Signers plus at least 8 Security Council members). Upgrades
+cannot take effect until both parties reach their thresholds.
+
+**Member selection criteria**
 
 - Representation across diverse geographic regions and international territories
 - Strong alignment with [Base’s mission and values](https://base.mirror.xyz/jjQnUq_UNTQOk7psnGBFOsShi7FlrRp8xevQUipG_Gk)
 - Diverse organizations - each member represents a separate entity
 - Proven track record in the Base and Ethereum ecosystem - in good standing in upholding professional and ethical standards in the community
 - Technical competency and good security practices - has completed screening processes, including background checks, and have shown ability to securely store and use sensitive key materials
 
-**Current Roster**
+**Current roster**
 
 This is a living list that will stay up to date with membership. As of February 2026, the Security Council currently contains members from the following entities and individuals, based in the listed geographical jurisdictions.
 
@@ -91,14 +97,14 @@ This is a living list that will stay up to date with membership. As of February
 
 Individuals representing each entity are not published to protect personal privacy and to enhance security.
 
-**Member Terms**
+**Member terms**
 
 The Security Council for Base operates on a staggered “cohort” model:
 
 - Cohort 1: current term lasts until October 2026
 - Cohort 2: current term lasts until January 2027
 
-## Roles & Responsibilities
+## Roles & responsibilities
 
 **Review and approve changes**