Somebody else can tell you better than I can, since I know zero about training to race for both disciplines.

But I will say that taper is taper, and if you're serious about your MTB race, then you don't want to be ramping up or even doing much running during your taper.

On the other hand, cardio fitness is also cardio fitness, and I've found that regular cycling allows me to run much longer distances on much less weekly mileage than most running-only sites recommend.

I'm a little bit in the same position with having let myself go earlier in the summer, and now training for a mid-October marathon. What I've done is to jump back in and see how it feels (so far, okay). I think that if you do a test long run today or tomorrow morning, at an easy pace, just to see where your fitness level is now, if it were me it would be far enough out that it wouldn't hurt my taper too much. That would hopefully give you some feedback on where you are now and what it will take to ramp it back up. Then just incorporate a very little bit of running into your taper for the next week - none in the days leading up to your race - just to keep it in muscle memory. Then start ramping up the mileage after your race.

But if I were you, I also would not take advice from someone who has a hypothetical magic 8-ball for her own coach.