F5 equivalent of Alteon's buddy helthchecks

Sounds like an "Alias Address" health monitor configuration. Normally you would create a health monitor, say HTTP, and apply it to a pool. In your situation you could assign it to the server A pool member and then create a new HTTP custom monitor with the Alias Address of Server B. Assign this to the member too and you could change Availability Requirement to All. This way if Server B went down this monitor would fail and take Server A down too.

See the following and Alias Address section: Monitors Concepts

Hope this helps,

N