What order will actions trigger in?

Personally, I would modify the triggering conditions for the two actions, to guarantee that they would occur in the right order. Something like this ...

Action 1 triggers when record 1 is created with field X filled. The action creates record 2.

Action 2 triggers when record 2 is created. This action clears field X in record 1.

Exactly how you do this depends on some details, such as whether the two records are in the same table or different tables. If in different tables, you will need each table to have its own report-link field to the other table. If in the same table, you could create a field called something like "source ID". It would be null in record 1, but it record 2 it would have record 1's Record ID# (written as part of Action 1).

I suggest an Automation as for sure you can control the sequence.  I have the perception that an Automation take a bit longer to run (just a feeling that it take a second or two to start, where an Action/ webhook is very immediate), but you get to for sure control the sequence.

I once had the opportunity to ask that same question face to face with a webhook developer at Quick Base and she said you could not guarantee the sequence as they could go into different queues.