Likehai,
I've finished a cursory peruse of your paper and find it very interesting, and certainly don't understand alot of it. But I do now understand what you mean by 'decoupling'. And I see, in clarification, that the primary intent of the research is to minimize HAZ impact while maintaining high levels of weld metal deposit (which has been possible for some time from tandem migs).
But, in answer to your original inquiry, I don't have an answer. If it is critical that the GTAW arc occupies the same space as the GMAW arc then I'm not sure technology is available (at least on a practical welding level)to control the arc blow. The only thing that comes to mind is pulsing. A technique used by tandem to eliminate electromagnetic interference. Perhaps this is more primitive than your own discussions, but if it works for two mig arcs why not a tig/mig arc apparatus. And the technology is certainly available.
On the other hand, what Dave had mentioned may work after all. This technology has been around a long time and is used commonly with auto/machine hot/cold tig applicaitons for simple arc oscillations. But maybe if you increase the amplitude of the magnetic field it will act to inundate, stabilize and override the variations of the two seperate GMAWE/GTAW fields.
WOW. Talk about specualation.