For PMP I’m providing on application three years/36 months unique non-overlapping professional project management experience.
When applying for PMI-ACP I’ll be needing 12 months agile + 8 months agile.
I’ve enough project experience to satisfy both - for example one ‘experience scenario’ is that I can use 2020-2021 two years working on waterfall and agile projects for PMI-ACP and use experience 2017-2019 to provide the 36 months for the PMP application.
The problem with that scenario is that my PMP experience on application will be from 2017-2019. Somehow I don’t like that idea as my experience seems ‘stale’ (like I haven’t been working in Project Management) when in fact I’m just deliberately using older experience to save recent work for a later application.
My question is - can I use my 2018-2021 experience on PMP and reuse that experience on PMI-ACP application or should I hold back / not use my 2020-2021 experience on my PMP?
Assuming you get your PMP first, you can use that to satisfy the "general PM experience" requirements. Then, you'd only have to find sufficient experience for the agile-focused PM experience.
No issue with the timeframes you have provided - PMI allows experience to be from up to eight years back.