I do this and it works great. I use a second computer on my wrist as the primary and it stays with me between dives.. Both are Sunnto and use the same algorithm. I put the larger Vyper on the arm, so I can see it with my poor eyesight:) It's like having your speedometer in your car, right in front of you.
The Vyper nd D6i are set on the same channel with the pressure sensor so both give me my gas status. I also have a small SPG stowed along my BCD so I also have redundancy here on gas supply. This has saved me on a few dives when the sensor and computers stop communicating.