You can often manage it without tying the packs together and instead threading the waist belts of the packs though the shoulder straps - probably not as secure, but worked well with no problems over a short distance on ML.
Thanks for all the advice about equipment. Just got back and wanted to give a quick update. Firstly, Tenerife is awesome! Absolutely incredible landscape and wildlife.
We spent the first day travelling across the island up to the caldera near Teide which was an amazing landscape. I was also in the remote north-west, which had some beautful remote valleys looking down to the sea. (Paddy, we didn't actually go down the Barranco de Masca - maybe next time?!)
With regards to gear: I wore long sleeved shirts (Karrimor, from Sports Direct), which did a great job keeping off the sun. For trousers, I picked up a pair of Ayuacho Duno zip-offs, which were suitable lightweight. A big broad-brimmed sun hat was also very useful.
I have both a Berghaus Freeflow and a Deuter Futura which usw the trampoline style back. I didn't actively seek them out - it was just a feature that they happeneded to have. I think that they do work well and certainly seem to keep my back cooler than other rucksacks I've tried. That said, the normal cushioning type doesn't seem to be that bad anyway.
However, the downsides are that the pack is slightly less stable (further away from your back) and that they can be quite a bulky system, taking up space.
I've not been able to use it outside yet - I'm off to the Brecon Beacons in April, so I should be able to report back then.
First impressions are that it is quite long and roomy. It's the right length for me (I'm just over 6ft 1) but i'm quite skinny so it's not too tight either. It lofts up nicely and compresses down very small. It certainly feels very warm too, although that's from trying it inside the house.