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.