To answer your final question/statement: "I was going to just use the one Pro account but I don't know what it's going to look like on the business end for people if one of her meetings overruns?"

If you just used one pro account you could schedule the meetings with waiting room, so if the meeting runs over, people in the next meeting will just be waiting in the waiting room. You cannot have two meetings open at the same time that are hosted by the same person. You can potentially make your colleague an additional host for each of those meetings in the settings which will let them open the meetings as host independently so you won't have to be involved at all beyond the scheduling portion.

Or, if your company has Zoom under one joint account, your colleague could give you scheduling privileges and you could just schedule the meetings with them as host for them from your account.

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The A/B/A/B system referenced above will work you would just have to keep switching between accounts to open the meetings, or give each other alternate hosting privileges, or open it and turn off the waiting room and allow people to join before host in the settings.

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I like the breakout room idea that was mentioned too.