Like Greg, I signed the first one and not the second one.
However, had I been asked to sign the second--I wasn't--the only reason I would have refused is that it's billed as "Economists for Trump." I almost never sign a letter where the signers are taking a position in favor of a politician rather than in favor of some of a politician's policies and positions. But the author(s) of the Trump cleverly came out in favor of Trump's good policies while expressing their hope that certain things would or wouldn't happen.
Here's the key section expressing hope:
We believe that reciprocal free trade with lower trade barriers on all sides produces higher overall economic growth and hope that the President's efforts to negotiate better trade deals, including willingness to re-enter the Trans Pacific Partnership (TPP), will bring about a stronger long-run free trade equilibrium by reducing trade barriers and opening markets that were previously closed to American businesses.
We hope that Chinese President Xi's stated willingness to reduce China's 25% auto tariff will be honored and is an early sign that President Trump's negotiations to promote trade are working.