A heated screen expert I spoke to recently (about custom-made screens for a different vehicle entirely) said that splitting the elements into two sections like this enabled a faster warm-up.
However - it came with a warning, in that heated screens - especially of this type - should be wired thru a timer relay so that they are turned off automatically after a short period. If they're not, and are left switched on for extended periods, the elements start to break as they effectively over-heat.
This echoes my own experiences in the 90, where I gradually get more and more foggy stripes on the screen that either take a long time to clear or don't clear at all. I'm now on my second screen - needless to say despite best intentions it tends to get left on constantly at this time of year. I'll shortly be exploring the warranty situation (bought from main dealer, but installed myself), though I haven't found a suitable timer relay yet. Anybody got any ideas, pls let me know!