Can I run two WhatsApp accounts (or two of the same app) in Spanbox at once?

Yes, multiple accounts of the same app is basically the whole point. Each account you add runs in its own isolated session, so your personal WhatsApp and work WhatsApp sit side by side, both logged in, both getting messages, no logging in and out. Same for two Slack workspaces, two Gmails, two Discords, whatever. They don't clash because each has its own private session on your machine. This is the thing WhatsApp/Slack themselves make painful (WhatsApp officially only wants one web session), and running each in a separate Spanbox session is the clean way around it.

This sold me. Two WhatsApps, three Gmails, a couple of Slack workspaces, all live at once in one window. Every account isolated so none of them log each other out.

Add each account as its own entry and they each keep a separate session. The unified inbox then shows unreads from all of them together, which is the nice part, one list across every account.