Welcome to the land of dual sim, my requirement since 2003. It's incredibly useful even if only professionally.

Up until earlier this year this was easy. Any sandboxing app like MultipleApps would load WhatsApp and then you'd have a clean break if you chose. But Zuck decided to block all non original instances, giving a warning for about 6 months that it wasn't supported and bad and lies about stealing your data, then just logged out and that was the end.

Now very few sandboxes work and in my experience it's only the root level ones that come with your device. From there you can block contacts or permissions for WhatsApp.

But I think you've got it backwards as you'll be the one with the mixed contacts. Your work colleagues won't have your personal number and your personal colleagues won't have your work number. And if they did, it would message two separate WhatsApp instances. They wouldn't see your other contacts, it's not like Outlook.

And in that case a free Google voice number on WhatsApp's second account in the same app, would work just fine for you. That's how I have 4 WhatsApp on my 1 device. 2 physical sims always on, plus google voice, and an esim. Each WhatsApp instance with a second account switch and then the second WhatsApp in my phone's sandbox.