It depends on the conventions you use for reading your dice.

You have a percentile dice (or a d10 you nominate as your percentile dice) and a d10 dice.

There are broadly three ways to read these dice, each with their own conventions. So long as you are consistent with your convention then any of these methods are functionally equivalent.

Method 1 - read the d10 as 0-9 and the percentile dice as d10*10 (ie 00, 10, 20, ... )

The natural sum of the two dice then gives you values from 0 - 99. This method won't be suitable for games like D&D (which want a range of 1-100)

Example Calculations:

Method 2 - read the d10 as 0-9 and the percentile dice as d10*10 (ie 00, 10, 20, ... ), with a value of 00, 0 being 100

With this method we've introduced an exception for a roll of 00, 0. In particular, we've removed the lowest value from the previous set of sums.

Aside from the exceptional case we sum values on the dice as in method 1.

So now we get a range of 1-100 on the dice, with the lowest possible roll being 00, 1 giving a value of 1.

Exceptions:

Example Calculations:

Method 3 - read the d10 as 1-10 (assigning the 10 to 0) and the percentile dice the same way as method 1

In this method we do a straight sum of the two dice to get a read.

This gives us a range of possible values of 1 - 100, with the lowest value being 1 (on a roll of 00,1)

Example Calculations: