Impact of Potential Clearance Decision with One Agency on Current Employment with Another?

Seeking thoughts on the following scenario:

David presently works for DoD with a TS/SCI clearance (Job #1). He is also currently processing for a job with another agency requiring a new clearance, also SCI (Job #2).

While he likes Job #1, if given the chance, David strongly prefers to leave for Job #2; its long-term career prospects are better and far more appealing in his view.

However, the process for Job #2 has dragged on for nearly three years at this point! Between the time he received his clearance for Job #1 and started processing for Job #2, he has had extensive foreign travel and gotten married to a dual national woman (U.S./Western Europe) with a foreign national father. He assumes these developments have contributed to this slowdown, but had not expected to still be processing by this point.

Given this, David is concerned that the process for Job #2 might not materialize, and that an adverse decision could affect his current employment with Job #1. As much as he wants to work for Job #2, he does not want to lose his continued prospects for cleared employment.

Curious about how the experts would advise him in this situation. Many thanks.