does "Joiner Adept" allow you to tap each land card for two mana?
No. It adds a second tap ability to the land, and you cannot tap it twice for both mana producing abilities your lands would now have.
For example, say you have a swamp.
Normally a swamp implicitly has
"{T}: add {B} to your mana pool. "
If you add {Joiner Adept}, your swamp would look like this:
"{T}: add {B} to your mana pool.
{T}: add one mana of any color to your mana pool."
You can choose either one you want, but once it's tapped, you can't tap it for the other option.