That's "a way" to do it, although there are a number of differences that might make you choose to keep separate layouts for iPhone vs. iPad. Usually I find that my layouts for iPad mimic simplified versions of the desktop, while the layouts I make for iPhone are usually button driven "slides", so I end up with two completely different versions. If you're looking for the fastest way to support multiple iOS devices, you may have found it.
Problems? Well as long as your objects are "smart", and have anchoring to stretch and contract them, then for the most part it should be ok. I'm not sure, but I thought locking the zoom level prohibits pinch and zoom gestures so that's something else to keep in mind.
Rather than triggering it to run on every layout, it might be easier to set the detection and zoom lock in the login script.
Also, thorough testing of it on both devices will eventually lead you to the conclusion of the acceptability of the UI for your needs. Might want to hand off the app to a non-technical user though and see what they think (white box testing).
thanks Mike, I think it would certainly "let me away quickly" but as you say it may only serve the most basic requirement's. Great..
I have 1 Layout with approx. 640x970px dimension.
Startscript for iPad is Zoom = 100%
Startscript for iPhone is Zoom = 50%
Much can be done with setting Anchors for objects. Stretching with Anchor Left AND Right, or Anchor Down, can compensate some gaps given with different dimensions. Though you can even deal with differences between iPhone 4 AND iPhone 5!