Ooo... actually, maybe not. I have a DX6i, it's what I started the hobby with, but unfortunately, I found I had to quickly move away from it when I found that the limited two mix setup with a borked built-in elevon/vtail mixer (which didn't give full range, if I remember correctly) meant that I couldn't setup something more complicated. It's been a while since I had to setup a complicated mix on the DX6i, but it was too frustrating and limited based on their trying to oversimplify the UI.
If you can setup elevon mixing on the DX6i while mixing the elevator stick output, within the two mixes, it should be possible, but I have a feeling the mixer setups were also pretty limiting. Like it's a 1:1 channel thing or somesuch.
Anyway, another option would be as you thought, to use a V tail mixer or even a small programmable flight controller, but that's probably way more complicated.