Very sad news. RIP, and thoughts with those who remain.
Could someone flesh out the benefits and risk matrix on having a second helicopter involved?
Can't give you a matrix, but top cover is always nice to have when well out to sea for the reasons given. I think I'm right in saying that the RN used to send a chase aircraft routinely when the 1sts aircraft was heading out to sea. RAF SAR tended to use the Nimrod for long range jobs because it could get there quickly, pinpoint the vessel's location, drop stores a/r and stay on scene for ever. The risk in SAR is normally with the aircraft doing the winching (specifically, the winchman) so the top cover aircraft isn't generally exposed to any unusual risk.