[rtkwe]

Google Voice w/ SMS in Hangouts might fit your use case. I've never tried it with multiple cellphone numbers so I can't say how well it handles that. It does send from your Google Voice number so in theory it should be able to deliver message sent to you to multiple devices and keep your sent messages in sync. I know my sent messages appear just fine in multiple phones and on the web interface.

Better it works pretty seamlessly for people not using it because it just appears to come from a regular phone number to them. They just have to have that number also entered in their phone. That's one of the major hurdles to having a good replacement for SMS is that if it doesn't integrate into normal SMS there's a friction if Alice doesn't have a phone that can use the fancy new SMS replacement that Bob and Carol are hooked on.

edit: I see someone beat me to this suggestion.