Unfortunately, for now, there is no way to send a SMS to multiple recipients from a web app on iPhones with the sms scheme.
I believe a workaround is possible, if you specifically know the audience of your webapp. You could require the installation of a native app with a custom URL scheme, that creates the SMS upon request.
This app could register on multiplesms://
, read multiple phone numbers, and create the SMS (I think this is possible from different applications I've seen on the Internet, never used them though).
So in your webapp, you could write:
Two numbers, no body text
.
But remember you would have to require the installation of the native app on your users' iPhones.