How does the 2nd Noble truth explain the cause of dukkadukkha?

Cūḷavedallasutta:

Miss, how sukkha-vedanā clearly appears as sukkhā, and dukkhā?

1. Āvuso visākā, sukkhā-vedanā clearly appears as sukkhā when it's arising, and dukkhā when it's changed (vipariṇāma;vanish).
2. Dukkhā-vedanā clearly appears as dukkhā when it's arising, and sukkhā when it's changed.
3. Adukkhamasukhā-vedanā clearly appears as sukkhā when it's arising simultaneously with vijjā, dukkhā when it's arising simultaneously with avijjā.

Note: Above sutta talking about the clearly appearing, so the answers mentioning just the adukkhamasukhā-vedanā, which harder to appear clearly than the other vedanas. But naturally, avijjā can arise with every vedanā in S.N. Saḷāyatanavagga, nirāmisasutta.
In paṭiccasamuppāda, dependent origination:

1. Every steps are arising simultaneously with phassa, vedanā, saññā, cetanā, jīvitindriya, and manasikāra in S.N. Nidānavagga, Samanupassanāsutta.
2. Also, every taint steps, craving & aversion, are arising simultaneously with avijjā, in Samanupassanāsutta, too.
3. So, caving causes the first Noble Truth, such as dukkadukkha. Because avijjā arising simultaneously with taṇhā, sukhā/dukkhā/adukkhamasukkhā vedanā, and bodily/verbal/mental apuññābhi-saṅkhāra, that causes, not simultaneously, The first Noble Truth, bad resultants (dukkha-vipāka) which arising simultaneously, such as viññāṇa, nāna&rūpa, saḷayatana, phassa, dukkhā (dukkhadukkha)/adukkhamasukkhā vedanā, etc.
4. But, ariyamagga causes the third Noble Truth. Because vijjā arising simultaneously with sukhā/adukkhamasukkhā vedanā, and bodily/verbal/mental puññābhi-saṅkhāra, that causes, not simultaneously, saupādisesanibbānadhātu, ariya's life, which leading to anupādisesanibbānadhātu, the third Noble Truth.

See the path of purification, CHAPTER XVII — THE SOIL OF UNDERSTANDING (CONCLUSION): DEPENDENT ORIGINATION, for more detail.