Following the White House's initial defence that the original whistle-blower's report was a single, second-hand account of a meeting and therefore unreliable, the coming forth of a second whistle-blower could be vital.
"In order to impeach Trump, 2/3rds of the Senate must find Trump guilty. That's a problem when a majority of the Senate is Republican.
Keep in mind however that in 1993 the Supreme Court ruled that impeachment was a purely political process. This is important because Nancy Pelosi has refused to have the House of Representatives vote on beginning impeachment proceedings, despite the White House publicly calling for her to do so. Moreover, in order to impeach Trump, 2/3rds of the Senate must find Trump guilty. That's a problem when a majority of the Senate is Republican.
Pelosi most likely knows that those Republicans will never vote for impeachment without rock-solid evidence, and even then they may be cautious about returning to their voters and trying to explain why they just locked up the man who those voters support.
What seems most likely to happen for me is that the Democrats will drag this out as long as they can. With the Democratic primary, 18 Democrats are tearing each other apart while Trump coasts into 2020 unscathed. The threat of impeachment will remind voters of the endless Mueller investigation while turning attention away from any scandals on their side.
This second whistle-blower could be important, but the Democrats first must decide whether they want Trump impeached and if it is possible, or whether they just want a political weapon.