I would've but didn't have the headroom..if you have chance to fit both at a little extra cost its worth it in the long run than to find its not as good and having to damage to fit the stuff
for what i see its worth doing and the extra insulation wouldnt be any harm in the long run infact it will improve its insulation value
The triso I used was the only multifoil that my bco would approve... It really is good but I made sure it was done correctly, well sealed and air gaps both sides.. warm in winter and cool in summer, well the walls are! I do however regret fitting two large veluxes in each bedroom...fine in winter but even with thermal blinds they feel like radiators in the summer! ...walls cool to the touch but the heat radiating from the veluxes is a problem!