How can I add extra documents to my multiple entry tourist visa application if it's still under review?

You e mail them the extra documents

You cannot apply for two visas at the same time if the application isn’t approved or denied before you travel. If you hold a passport from the list of 93 nations eligible for a 60-day visa-exempt stay you can first enter Thailand under the visa exemption and activate your visa later by a border run once it has been issued.

If your visa application is denied, the alternative is to use land transport companies to make border crossings in order to obtain new visa exemptions.

Extra documents are often provided by email so check to see if that is what they asked for

Can you email the document? Or is that not how it works.