In order to answer this question, it's essential to understand the concepts of atomic structure and nuclear physics. We know that stable atomic nuclei are usually identified via the 'magic numbers' which correspond to numbers of nucleons (both protons and neutrons) in a nucleus, making it more stable. The stability of an atomic nucleus is characterized by the forces inside the nucleus.
To find the smallest value of 'a' for which there are two stable nuclei, one approach can be to look into a nuclear and atomic data table. These tables usually provide information about atomic number, element symbol, mass number, isotonic number, neutron number, and more importantly, whether a particular isotope is stable or not.
Here, in the fourth column, you would look for atomic number (which might be represented as 'a' in this case), and in the seventh column, you would look for the stability of the atomic nuclei.
Once you scan these columns appropriately, you could potentially find atomic numbers for which there are two stable nuclei.
Remember, when two nuclei are considered stable, this typically means that they do not readily undergo radioactive decay, and therefore they tend to remain in their current state over a long period of time. The smallest value of 'a' you are looking for would be the atomic number of the lightest element that has at least two stable isotopes according to the data table.
It is crucial to note that computer programming or Python is not competent to answer this particular question without actual atomic and nuclear data. This requires actual examination of nuclear stability and atomic data, and not simply numerical or computational operations. Therefore, the smallest value of 'a' for which there are two stable nuclei could only be determined with detailed knowledge of atomic structure and a concrete atomic and nuclear data table for reference.