I am planning to go to Guilin and Zhangjiajie for two weeks.

September or October is the test time to travel in these places, neither cold nor hot. However, remember to avoid the National Day Holiday (Oct.1-7). It is suggested to spend 5-6 days in Guilin and 5 days in Zhangjiiajie. From GL to Zhangjiajie, there is no direct bullet trains and you can take a flight and transfer in Xi'an. The following itineraries offer to you for a reference:
GL:
Day 1 Arrive at the city and stroll around Zhengyang Road to taste snacks .
Day 2 Elephant Trunk Hill, Wave-Subduing Hill, Wave-Subduing Hill
Day 3 Take a cruise from GL to Yangshuo in the morning to enjoy Li River and visit West Street in the after noon. At night, you can enjoy the Impression Sanjie Liu
Day 4 Yulong River, Silver Cave
Day 5 Back to GL and go to Longsheng to enjoy Dragon's Backbone Rice Terraces.
Day 6 Depart to Zhangjiajie

Zhangjiajie:
Day 1 Zhangjiajie Forest Park, Yuanjiajie Scenic Area, Tianzi Mountain Nature Reserve
Day 2 Golden Whip Stream, Ten-mile Gallery
Day 3 Zhangjiajie Grand Canyon with the Glass Bridge, Boat ride on the Baofeng Lake
Day 4 Tianmen Mountain, Glass plank road,Tianmen Cave
Day 5 Fenghuang Ancient Town