Using both cameras on android: "Fail to connect to camera service"
Both cameras can be launched together as long as the hardware supports it. First you need to find the specific SoC used by your phone. You can find this by using the command
$ adb shell getprop
Chech the field [ro.board.platform], For Nexus 6 it returns
Now you would need to check if this chipset has dual-ISP capability in hardware. For Qualcomm snapdragon chipsets you can find it here. For Samsung Exynos you can find it here. Now if your device supports dual camera and has latest OS installed in it, then you can use this application (ref) developed by Jens to verify if the dual camera works. It should probably work, it does work on my Nexus 6. Video: http://youtu.be/lt8N1Lpa9Zw
This question basically says it cannot be done, but there may be some hack. This lack of response from google groups tells me most likely not. Here is another no-response post on google groups. Here is another post discussing the use of two cameras at the same time.
So my initial answer is no, it is not possible. BUT there may be some workaround, you just may have to find it yourself because it isn't discovered yet :).
EDIT 11/21/11: Unfortunately I found even further proof that it cannot be done, official from Sprint/HTC here. I realize you are using a Sensation, and this is an EVO 4g, but I'm sure it still applies.