By Mubasher Bukhari, Parisa Hafezi, Asif Shahzad and Ariba Shahid ISLAMABAD/DUBAI, Sept 11 - For much of the past year, Pakistan has played a dual role as Riyadh’s defence partner and Washington’s backchannel to Tehran but this week’s fighting between Yemen’s Iran-backed Houthi movement and Saudi forces is increasingly testing how long Islamabad can hold both positions at once.