In Sean Paxton's write up of DFH 120 clone he added the wlp099 on day five. Seems that for this yeast that mid fermentation is the proper place to use it in order to limit some of the esters.
Why not make a 16% beer? As long as ibus are calculated for the final beer getting the utilization in the original 1.070 wort won't be that difficult. I currently have a 14% RIS bottled up that's very enjoyable. I don't drink a lot of it, but it's a great beer. Was it hard to brew? Yes, but absolutely worth it, and there's no way I'd trade it for twice the volume at half the beer.
diety, I wish you luck with this and say totally go for it. Probably the only things I'd change would be the selection of your second yeast, and the timing of the DME additions. Probably do a pound per day or two as opposed to three pounds every five days. Check out this link about the 120 brew, may not exactly be the style you're going for but has good info about feeding the fermentation.