PaulDavis
This depends on the location, where d9780 lives.
In Europe, the most countries doesn't allow to buy and activate prepaid tariffs anonymously.
There is only one possibility: buy an eSIM data-only tariff actually for roaming (offers for travellers) at a very much higher price as normal SIM-only tariffs.
d9780
Conclusion: If necessary, your solution with two devices is much better than a not really anonymous (e)SIM with its disadvantages.
And because we all need an official phone number for more or less rarely cases (doctor, craftsman, shopping, delivery services etc.), best way is to use one official phone number and keep the rest of your data very private (and use separate email addresses/alias addresses).
You can communicate anonymously via messengers only.
Attention: When making calls via Messenger or sharing links, the IP address can become visible, so you have to be extra careful (telephone connection only via relay, do not switch on link preview).