Am I able to add a data-only sim-card to my dual-sim smartphone? I've a OnePlus 3 which has dual sim functionality. Of course I've also a single common smartphone sim which provides me with a telephone number and internet access. 

Since there are pretty cheap data-only sim cards (they are commonly used for laptops, tablets etc.) I wondered if my smartphone is able to handle such. It would increase my speed, coverage and data volume for just little money. Also I don't have to deal with two telephone numbers. 

Does this actually work? Is it a good idea?
Data SIMs are no different from regular SIMs regarding data connection. As long as your phone supports the configuration you want (e.g. many phones only supports Slot1 4G + Slot2 2G configuration - confirm whether you still need 3G/4G on the call SIM and place in slot accordingly), you can just pop one in and enjoy.