Will iOS developers need to constantly update 2 versions of their apps after releasing iPhone 5?

Not always, but some times

Its Depend on Your Client and The New OS Release.
to Make Application Alive you need to remain update.
and give support to latest Versions of OS.

I thnik it's much dependent from Your client.
In most cases it's important to keep apps up to date not only with device version but also with iOS version.
iPhone5 is using iOS 6 and Apple is going to release new iOS 7.

For developers ... "No need to update app :D"

For client ..... " Every second need another update ;) "

From my experience you should test your applications on every new release of iOS.
This way you will be confident that your application is working fine in new iOS as well, rather than waiting for customers feddback with issues :).
Remember single bad review may destroy your app, and you will see major decline in sales.