And How to go with installaing nvidia drivers for two different cards on the same OS (Windows and gonna use nicehash for starters...) ??...
Download the latest drivers for RTХ 3060ti. This driverpack should contain drivers for the RTХ 3070s.
1200 watts is enough, the main thing is that your power supply has enough connectors to power video cards and raisers.
RTX 3070s may perform better on other algorithms when compared to RTX 3060