Assuming the probability of dying in each activity is independent, planning to do a 7000-micromort activity A and then (after appropriate recovery time) an 8000-micromort activity B is not an overall activity with 15000 micromorts of risk. (The math is close enough when talking about single-digit micromorts, but it doesn't work in general.)
Pr(dying when doing A followed by B) = Pr({doing A and dying} or {doing A and surviving and then doing B and dying}) = Pr(doing A and dying) + PR(doing A and surviving and then doing B and dying) = 7000 / 1000000 + (1 - 7000 / 1000000) * 8000 / 1000000 = 14944 / 1000000
The larger the risks or the longer the series of events, the less accurate the assumption of linearity. This also doesn't take into account the fact that dying in extreme activities is not independent; by failing to die in A, you've demonstrated some small amount of survival fitness.