Joiner Adept

[Ulyses317]

does "Joiner Adept" allow you to tap each land card for two mana?

[Gorzo]

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.