SRT access with a 'backup' is one of the latest debates in tree climbing. Some say, definitively 'That will never happen… ever!'Have a look at Steve Ibelings 'rescue' at the 2011 European ITCC in Vienna using such a system. We are not exactly sure of the backup hardware employed but it looks like the Petzl ASAP - perhaps Steve will comment (he's a regular 'commentor' on the Treetools blog).
You can download the 2011 European ITCC results to see how Steve fared with this new setup. While you are at it you should check the scores for Taylor Hamel (of Treemagineers video fame) and Ben Abeleven (Australian Treemagineers product distributor).
Thanks to linwahling for this video link