According to Hanafi Fiqh, it is compulsory to offer all the three rakahs of witr salah with single salam. It is not lawful to offer it with two salams. However the preferred saying, in case it is offered behind the imam of other School of Fiqh, is that the faith of muqtadi shall be taken into consideration i.e. if it is offered behind such an imam who leads all the three rakahs of witr salah with two salams, then since it is not lawful as per the school of muqtadi who follows Hanafi Fiqh, and witr was not offered like this; hence for the Hanafi muqtadi it is not lawful to offer witr salah behind such an imam. In such a case he should offer witr salah alone.
However since some senior scholars of Hanafi Fiqh such as Allamah Hindwani, Abu Bakr Jassas Razi and author of Nihayah etc (may Allah have mercy upon them) have chosen the opinion that in case one is offering salah behind such an imam who follows other School of Fiqh then the school of that imam shall be taken into consideration. Hence if in the month of Ramadan, someone offers witr salah behind the imam of Kabah with two salams then his salah shall not be invalid as there is a big gathering of Hanafi muqtadis, so in this case if jamah is left and offered alone then it shall create problems, yet it is better to re-offer it later on as precautionary measure.
وقال ابن الشحنة : فالحاصل أن قاضيخان قال فى فتاواه : لا يجوز الاقتداء بمن يقطع الوتر وكذا فى الفوائد الظهیرية لأن المقتدي يرى أن إمامه خرج عن الصلاة بسلامه ومبنى الخلاف على أنه المعتبر رأي المقتدى أو رأى الإمام وعلى الثانى يتخرج كلام الرازى وهو يقول الهند واني وجماعة وفي النهاية انه أقيس (شرح منظومه ، 1/ 62 ، 63 ، شعر: 63 ، ط ديوبند)
Allah knows Best!
Darul Uloom Deoband