What is the minimum surface interval for a dive that is 70 feet for 29 minutes followed by a dive that is 50 feet for 39 minutes?

There is no minimum interval. A dive directly from 70 fsw for 29 minuts can immediately be followed to a dive to 50 fsw for 39 mins, and would be classed as a "Combined Dive"

To calculate decompression from this Combined Dive, is by taking the depest depth, (70 fsw), and total time (68 mins), and decompress on an appropriate table.

Using the USN repetitive dive times, this varies on the residual nitrogen remaining in solution.