The SendSMS command line does support sending to multiple numbers.
By design, when multiple numbers are specified, SendSMS will NOT wait for the messages to be sent. It will therefore report “Message has been queued.”
You can override this behavior by specifying a timeout, for example:
SendSMS 65xxxxxxxx,65xxxxxxxx "Your SMS message text" 15000
This will tell SendSMS to wait 15 seconds for each recipient (e.g. max wait will be 30 seconds)