If using dual sim on Huawei P20 Pro would changing sims show in either Google Activity or Huawei Data Download I have a Huawei P20 Pro with dual sim capability. If I used (signed into) the second sim with another Google account, would My Google Activity on the Google account of the first sim show the switch, backwards and forwards, between the two sims? OR if not.. Would the switch be shown on the Huawei download data for my phone, if at all?
Google accounts are independent of the SIM card, you just need Internet for accessing Google. If this happens on your device with SIM1, SIM2 or Wifi is irrelevant to Google.

Note: Usually in dual-SIM Smartphone only one SIM can be active for Internet access at a time.