There seem to exist different versions of the phone.
I can insert 2 nano SIM cards in the device but how to activate the second one? There is no enter PIN dialog popping up. On YouTube I saw an unboxing video that showed dual SIM support.
Enter download mode by powering down the device and then holding both VOL_DOWN and POWER to power it back up. It will boot to a screen saying
hTC download mode.
The 3rd line of the displayed info will say something like
htc_oceuhl PVT S-ON.
uhl corresponds to the single-SIM variant,
dugl is the international dual-SIM variant, and
dtwl is the Chinese dual-SIM variant.
You can exit the mode by selecting "reboot" in the download mode menu.