trying to figure out how to upload a second 1099B for a second brokerage account. After I entered the first one, there doesn't seem to be another prompt?

After you finish importing a Form 1099-B, you should see a screen that says "Let's finish pulling in your investment income.  You can import more from more accounts by selecting Add Investments."  There should be a box towards the bottom of the page that says "Add Investments".  If you're not seeing that page, or that box try:

• Click on Search
• Type in "1099-B"
• Click on "Jump to 1099-b"

That should take you to the page referenced above.