Adafruit Ultimate GPS Logger Shield and Co2Meter (K-Series) together???

Hi All, I have an important question for my project. I'm using Adafruit Ultimate GPS Logger Shield on Arduino and at the same time reading Temperature/Humidity sensor from also Adafruit. It works well. Now I'm trying to add Co2 Sensor (CO2Meter.com) that also use tx rx but in pins 13,12. But after trying for many hours I don't find the way to make them work together. The problem I have is that the SD card can't create the file. But looks like both systems are fighting for the Tx RX port to send the information. How can I mix this two sensors??? (GPS and Co2)

Thanks a lot for the help

David

Which Arduino do you have? Which shields do you have? Post your code.