The English travellers were crossing deserts and jungles. (not only…but)
The English travellers were crossing not only deserts but also jungles.
The English travelers were not only crossing the deserts but they were also crossing jungles.
اي الجملتين صح.... ومن وجهة نظري ان الثانية هي الصح لان الفعل المساعد لازم يكون قبل not
برجاء التوضيح مع خالص الشكر والتقدير
محمد