You might try this one

Solved: Customer submits form and the new data is attached... - Airtable Community

It does require an extension which I've reached my limit right now, maybe I'll switch some things around and see if I can try this out.

You can actually do this without the extension, especially if you only want to prefill one or two fields (you can also hide the reference/record ID field).

Here's a video describing how to do this: https://youtu.be/_XPvXARpUo8.

Wow this is nearly perfect! Now I just need to figure out how to automate linking the second table back with the first and updating the record in the first table accordingly. Any ideas on how to do that?

Yes, that part is pretty simple!

After adding a linked field between the two tables, set up an automation that triggers when a form is submitted.  Choose the action, update record. Paste the value from the prefilled record ID field into the linked field and it should link automatically. 

I totally agree it seems like it should be simple! But I can't seem to figure it out and can't seem to find a good tutorial, any suggestions? Right now I putting an email field in both tables, so I could match those, but the automation process seems to look for a Record ID and I'm not sure what to do at that point, do I just pick the email field? Sorry newbie here, but a quick learner!

Another quick, but maybe obvious question: Is there not a way to do this with just one table and form? Does a form completion ALWAYS have to fill in a new row? Wouldn't it be easiest to create a form on my Contacts table that simply allowed the individual to update certain fields in their row entry?

Forms can only be used to add new records.  Check this documentation out: https://support.airtable.com/docs/use-case-update-records-via-a-form

I actually usually match by email (those tend to be more consistent than names), so you can do that. 
But I don't think you need to.  Take a look at this: https://airtable.com/shryorbYeeUnqt2x5  (Ignore the Dates table. The main thing to look at is the Automation.)

(If you do match by email, be wary of spaces inadvertently after the email. You can use TRIM() to take care of that.)