I'm not sure exactly what you are asking. You can install Messages + on your phone, and sync the messages to a tablet, a laptop, etc - all linked. You can use the tablet or the desktop app to schedule messages that will be sent from your phone number.
What is the "second device"? Another phone? A tablet? Is the second device using the same phone number as your phone?
Can you tell me the steps in the desktop/laptop version? I can't find the correct buttons... maybe some screen shots too would help!
Interesting - at a quick glance, I'm not seeing a way to schedule a future send in the desktop app or through verizonwireless.com. In the phone and tablet app, you long press the send button and options for scheduling pop up.
I've never tried it on the desktop and that doesn't seem to work. The only scheduling I've done is from the phone app. I'll keep looking and let you know if I find anything. In the meantime, maybe someone else has figured this out.