Interfacing GSM SIM900 and ESP8266 with Arduino

Hey, I am having a problem with my project. I want to interface the GSM shield and wifi module ESP8266 with Arduino at the same time. Can anyone solve my problem with their suggestions...

Please provide a detailed description of the problem.

Basically my project is to Request sensor data via SMS. But there is a problem when there is no mobile signal. So I prefer to connect the WiFi module and GSM module to the arduino at the same time. But I didn't get any suitable documents from internet.plz, give me some suggestions.