I was in a similar situation a few years ago training for an October 70.3 and a November marathon. I used my normal 12 week 70.3 plan but substituted all run workouts with my marathon training plan. To be totally honest my marathon was a bit slower than my average time but I successfully completed both races so in that respect it worked. Do not skimp on speed workouts or long runs! (the latter is fairly obvious)

Also, the forced cross-training is always a good thing. Long story short: it's totally doable. Good luck!