I'm trying to purchase a second subscription for a second company and sign-in to both companies by using my current sign-in info but when purchasing it won't allow it.

Hello, @arizonasunflower.

I'll route you in the right direction for support to help you fix the error you encountered while purchasing another subscription for QuickBooks Online (QBO).

The first thing we can do is double-check if you're using the same credentials for the company.

Otherwise, you may contact our Sales Support Team so they can further check your account and be able to guide you on the next steps. Click on this link and find their contact details in the upper right corner: Talk to Sales Support.

Once done, you can switch between your companies quickly by checking out this article: Switch between company files in QuickBooks Online.

I'll be around if you need further assistance managing your QBO subscriptions and accounts. I'm very eager to help. Take care.