I disagree with Empty Cruise. I think that during an emergency descent the chance of triggering an RA is MUCH higher. Your (necessarily) high rate of descent will bring many more targets below you into the "threat zone". TCAS does not know that you are going to level off at a particular level (typically FL100ish), so an awful lot of aeroplanes that would be ignored during your normal 2000fpm descent rate become important at your 5000fpm+ rate of descent.
We set TA only if an engine fails "to prevent climb commands which can exceed single engine performance". So there is a precedent for changing TCAS mode during abnormal operations.
The good thing about setting TA only is that conflicting aircraft will manoeuvre around YOU. You can just carry on unhindered with the emergency procedure you are already carrying out rather than have to start another one.
Who can say that a crew receiving an RA whilst they are carrying out an emergency descent will follow it? At the very least I would suggest that they would pause longer than normal whilst their poor maxed out brains compute the information they are being presented with loud airflow noise/uncancelled alarms/bleeding eardrums* delete as applicable. The last thing we want is crews manoeuvring against an RA, Uberlingen anybody?
Having said all the above, selecting TA only is not part of our emergency descent checklist.
G W-H