I tried to confirm Huawei support this morning. All the new dual SIM Huawei's support dual LTE as far as I could see from their settings menu. The problem with testing in-store, they tell me, is that they loose insurance on the phone as soon as a SIM is put in, LOL. Even the in-store Huawei expert had no clue, he claimed to be Sales and not Technical!!
I have contacted the Huawei Mobile Hotline number 0027 860 861111 as well as via On-line Chat with a nice lady called Nonnie. They both confirmed that VOxx on SIM1 (foreign for our purposes) using data on SIM 2 (local) was NOT POSSIBLE. I am not sure that they really understood as this is a bit of a niche question.
They did suggest that try their local service center to try to arrange a test device. I am waiting for them to call me back.
Ah well. It seems like it might just be an Apple thing. To be fair I had no idea until I saw @whatwhat’s post on the other thread.