Should I main Bard and Thresh in Silver 2

Main anything you want. Ignore the salty naysayers that want to decide for you.

Main whoever you want basically.

However, I'd warn you about Bard because Bard requires a team that knows how to interact with his ult.

no

Thresh yes.

Bard in silver I doubt it, most people don't know how to work with him.

In silver2 instead of focusing on "maining" a champion you should rather take a look at improving your general gameplay first. Especially since in silver garbage you can play anything along with ad janna and the likes.