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Dependent origination is the cycle of the happening event. Each event has only one decision. There are 2 saṅkhāra for each decision: wholesome and unwholesome. Wholesome saṅkhāra gives good resultants. Unwholesome saṅkhāra gives bad resultants. So, viññāna to vedanā which are the resultants of saṅkhāra must be singular, because it's just the resultants of one decision. Jāti and jarāmaraṇaṃ are same. Wholesome karmma-bhavo gives good resultants. Unwholesome karmma-bhavo gives bad resultants. So, jāti and jarāmaraṇaṃ must be singular because of the same reason.
Above explanation explained clearly in abhidhamma vibhaṅga paccayākāravibhaṅga and it's atthakathā. It separates each event to singular as "saṅkhāro". That's why no direct explaination in any atthakathā, included path of purification. However, it's used widely in difference context such as in VN.A. Introduction.
And this is the example of what I wrote "it's used widely":
Kusalattika: kusalā dhammā, akusalā dhammā, abyākatā dhammā.
The "wholesome" triad: wholesome phenomena, unwholesome phenomena, not both phenomena
The Path of Purification detailedly exposited the dependent origination follow to 'the "wholesome" triad':
jarāmaraṇaṃ = not both phenomena
jati = not both phenomena
bhavo = not both phenomena
upādānaṃ = unwholesome phenomena
taṇhā = unwholesome phenomena
vedanā = not both phenomena
phasso = not both phenomena
saḷāyatanaṃ = not both phenomena
nāmarūpaṃ = not both phenomena
viññāṇaṃ = not both phenomena
saṅkhārā = 1. wholesome phenomena, 2. unwholesome phenomena
avijjā = unwholesome phenomena
You can see whether saṅkhārā are separated into 2 events.
Above quote is my own conclusion. To read it, the reader needs to understand in advance of chapter XIV, XV, XVI, XVII, and some part of chapter I, III. Also, it requires Abhidhamma knowledge in advance, as well.
Path of Purification, CH. XVII, THE SOIL OF UNDERSTANDING—CONCLUSION, DEPENDENT ORIGINATION, III. Detailed Exposition page 547-608.
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Disappointing there has only been one attempt to answer this question. Often questions are asked about if only one sense organ can operate at the same time therefore I expected some interest in this question.
The impression is each link (except sankhara) allows for the common arising of one type of phenomena in a moment. For example, eye contact being the predominant experience or sensual attachment being the predominant experience or sensual becoming being the predominant experience.
However, with sankhara, the impression is all three sankhara (namely, kaya, vaci & citta sankhara) can arise together, at the same time.