Here's the current Zoom Support article that covers multiple simultaneous meetings:
https://support.zoom.us/hc/en-us/articles/206122046-Hosting-multiple-meetings-simultaneously
Please note the second bullet in the prerequisites section:
- Host must start the meeting
I'm not sure if you must start both meetings, but if there is a meeting in progress (whether you started it or not), then you must start the second meeting. You colleague can't initiate the second meeting by joining it, even if Join Before Host is enabled.
If you are on a Business Plan account, this should work for you. You mentioned you "have a business plan", and I'm hoping you mean Business Plan (capital letters) which is a distinct Zoom billing term for accounts with 10 or more licensed users, as opposed to "business plan" (small letters) meaning "a plan I use for my own business."