ORIGINAL: mydartswinger
I don't have an Alpha or a DX6, but I was able to buddy box with my DX7 as a master and an old JR sport 4 channel analog radio as a buddy box. I was unable to get it trimmed enough for hands off flight on the JR, but it was still manageable. The buddy box for a computer radio doesn't necesarily have to be another computer radio. I have attempted to use a Futaba as a buddy box, but for some reason the DX7 didn't like Futaba.
I may try contacting Spektrum directly to get an answer. It seems like you should be able to do this, but in the instructions is says that the DX6i can be used as a master and a slave but the settings have to be the same on both transmitters. I'm wondering if that's just when two DX6i's are used.