[Fred Hempel]

There is a 1/16 chance that you will get both traits homozygous. However, you don't need to grow out a large number of plants to make sure you hit the 1/16 lottery.

If you are working with F2 plants from an F1 parent that was heterozygous for both traits, one in four plants will be homozygous for either trait. Which means you have a greater than 1/4 chance to get a homozygous recessive for one or the other traits. You can pick any plant that is double homozygous (the recessive trait is evident). Even if you don't see the other recessive trait, there is a 2/3 chance that the other recessive trait is present as well (in a heterozygous arrangement).

So, you can grow 8-12 plants, look for double recessives for either trait and collect all of the single double recessives with a very good chance that the second trait will show up phenotypically in 1/4 plants in the next generation.