On 2a of bava kamma, the gemara asks what the practical difference is between avos and toldos of shabbas, and then answers that while two avos or two toldos (of different av categories) trigger two obligations, an av and a tolda (of the same category) trigger only one obligation. But don't two toldos (in the same av category) also only trigger one obligation? If so, how did the gemara answer the question? - i.e. ultimately both avos and toldos are exactly equivalent in that they cannot combine with other toldos in the same av category to trigger more than one obligation but they can combine with avos or toldos in other categories of melacha to trigger more than one obligation!?
Yaakov Saibil
The Gemara is not answering that an Av is better than a Toldah. It is just answering that it is categorized differently, which makes a difference in triggering obligations, as you stated above. Ultimately, everyone agrees that the categorization of an Av as being the more important Melachah and a Toldah being less important in the making of the Mishkan is why they are called Avos and Toldos. [This is indicated by the Gemara as well. First it asks, what is the difference between Avos and Toldos? It then asks according to Rebbi Eliezer, "Why is one called an Av and one called a Toldah?" In other words, if there is no practical difference, what is the reason for their being categorized as an Av or Toldah?]
All the best,
Yaakov Montrose