The tension issue never crossed my mind. I suppose if the the original stays are designed to have a certain tension or stretch value then you could not just add one more stay on one side and not over tension/stretch the other. I suppose it all becomes moot if the back stay will safely stretch enough to allow the solent stay to fully tension? Eg. if the stays normally run 2% stretch and you then tension a solent to the point where the fore stay is unloaded completely the back stay would now be at roughly 4%.
In practice I think you would find that the actual number would be closer to 3% as the fore stay might retain 1% plus the solent at 2% to balance out 3% at the back. DISCLAIMER - I PULLED THESE NUMBERS OUT OF MY A*S AND NO CALCULATOR WAS USED IN THE MAKING OF THIS THEORY!