In the DC case Trump's lawyers are arguing that he can't be criminally charged unless he's impeached and removed first.
In the Georgia case, Trump's lawyers are claiming that he can't be charged because he was already impeached for that action and that's double jeopardy.
Ignore for the second that the Georgia crime is not what he was impeached for. Trump's lawyers are arguing that you can't charge a President unless you impeach him, but also impeaching him means that you can't charge the President.
"I'd totally murder people, but that evil federal government would lock me up for murder? Can you believe the audacity?!!! I really want to murder these people, but they won't let me. Total government overreach! Whatever happened to states' rights?"