Mixing 60% and 81% dark chocolate to get 70%
Functionally, it should work out fine; but it is not an exact substitute. 

Based on the fact that your recipe gives you a tolerance for both strength and quality of the chocolate I would say it is probably fine to proceed boldly with your plan to mix the two. You might try a small sample melt first (as suggested here) by mixing an equal but smaller amount to check your results first.

Will this create 72% (or even 70.5%) chocolate, actually no. There is more to chocolate than just the %cocoa. % of cocoa is just one element to the formula. The 72% 'on the shelf' almost certainly has different ratios of sugar, chocolate liquor, butter etc. Once melted these would react differently with the other elements of your recipe if that recipe were more exacting in it's requirements.

There is an interesting collection of articles on the chemistry of chocolate here, but nothing indicates that you should face any problems from an inexact match of 60+81 vs. 72