I don't understand what you mean. I've only had dual SIM card (physical card) mobile phones for years and it's never an issue to have my Chinese SIM and a German one both in the phone at the same time. Of course, data usage is drawing from the chinese SIM in China.

Newer iPhones marketed in North America, Japan, Middle East and other places have gone SIM card slotless. The next generation of iPhones in Europe is expected to be slotless as well.

If this trend continues it might be back to carrying two phones again, or getting dual-number eSIMs from HK/Macau.

Well best acceptance is +86 for sure, but there are many services that allow +852 numbers but not +1 or other numbers, and so I think that's at least some value especially if we're saying the cost is only 50 HKD to bolt onto a 3HK Data SIM. Meituan delivery comes to mind in terms of being able to accept HK numbers.

I generally don't buy local SIMs for travel. China is the only exception because having a local number there is a necessity or near-necessity for so many different things. In other countries (and also in China when I need a roamable international SIM), I use a Roamless eSIM. Roamless works globally and has decent roaming rates. Most importantly, its flex plan has no data/balance expiration.

I don't buy local SIMs anymore in a lot of cases, but I've also had trouble with app-based eSIM operators before, so now I keep a small curated collection of SIMs/eSIMs from different countries with reasonable roaming pricing to use as needed. It also helps for occasions where I need to make voice calls.