Is using a script the only way to have a new entry in Table 1 trigger a check for an existing record in a Table 2 and if no matching id is found, create a record in the 2nd table?

Yes. But it would be better to provide more detail about what you are trying to do.

I store donation info in one table, and donor info in another, and compile a series of summaries (FY Giving, Annual Giving, special campaign totals, etc.) in a third. Typically the summary info gets created as a batch, but recently we want to track it on an on-going basis, so as a new donor gets added to the list, I'd like a new summary record to be created also. In the summary table I use the getsummary function so we can query on results based on giving levels.