First, you have the right to say nothing at all; there is no requirement to incriminate someone else to protect yourself.
Second, you can still generally invoke the 5th amendment during testimony even if you already claimed someone else did it. You aren't under oath until said testimony, so it still protects against you having to choose between committing perjury or self-incrimination, and doing so cannot be used as evidence of either.
First, you have the right to say nothing at all; there is no requirement to incriminate someone else to protect yourself.
Second, you can still generally invoke the 5th amendment during testimony even if you already claimed someone else did it. You aren't under oath until said testimony, so it still protects against you having to choose between committing perjury or self-incrimination, and doing so cannot be used as evidence of either.