Yes, we can use two JIo sim simultaneously in OnePlus 6T. Both the SIM slots are capable to support 4G with all the latest bands, so you can use any two 4G SIM easily.
The OnePlus 6T comes in Midnight Black, Mirror Black, and Thunder Purple colors.
To see the complete overview of OnePlus 6T :
To see the complete specifications of OnePlus 6T : OnePlus 6T