If Spanbox keeps everything local, can I use it on two computers (work laptop and home)?

Because it's local-first, each machine keeps its own sessions, so you install Spanbox on both and sign into your apps on each one separately. There's no cloud account syncing your logins across devices (that's the trade-off of not having a server, nothing to sync through). In practice it's not a big deal: you log into your apps once per machine and they stay signed in, just like your browser would on two computers. Your actual chat history lives with each service (WhatsApp, Gmail, etc.), so you see the same conversations on both, it's only the Spanbox setup itself that's per-device.