That arrangement would have three conductors surrounding the antenna. As they would presumably be without insulators should would make for a poor antenna.
Same situation with using shrouds, or as one poster above said an antenna hoisted alongside the backstay. Might work, but certainly not advisable.
You have a tough situation there. Difficult to predict.
Fortunately one only needs to emit a few watts to make contact much of the time. That is why many cruisers with less-than-textbook antennas report good performance. But sometimes one needs more power.
I think the textbook solution would be to insulate the backstay and the triatic. Then drive the base of the backstay with the tuner. The triatic will be an unconnected part of the antenna at the higher frequencies. If you have a wire topping lifts on the booms they should be replaced with synthetic line.
As one poster pointed out, the horizontal antenna will be somewhat directional, aftwards, this can be used to advantage by changing the heading to put the other station generally aft.
But who knows ??? Consider a sat phone