Comment by entrope

I can't speak to your specific setup, but if the two SIMs use different carriers then they will often need to use different frequencies. A phone could in theory use twice as many radios, but the emitted power limits would be the same, so you'd have reduced range or throughout for each (plus additional size, weight and power for the second radio). Access points have different SWaP budgets than phones.

When I travel, I try to use an eSIM that uses a network my home provider partners with. This seems to improve the chance that voice and data over LTE or 5G NR work simultaneously.