Providing your FG is stable, adding extra yeast (yes, even champagne yeast) will not further reduce it. So bottle bombs should not be an issue in this scenario. I think your plan is solid. Lalvin packets are only 5 grams and not the typical 11, so you could use the whole packet, but you may be better off rehydrating 1 full packet properly, then splitting the rehyrated yeast between both cornies along with your priming sugar for both kegs too.
A buddy of mine bottled up a strong saison aged in a barrel at his local brewery in this fashion with great results.