Well, not precisely at the same time: there is no "both" selection. You either see the results filtered by iPhone or Android; you can't see both. Oh, and that appears to be something else: it doesn't appear to search for iPad apps.
For example, we tried to find "The Daily." It's iPad-only, but doesn't show up when you search on Yahoo's page.
Also, the search is limited in terms of what it can do in terms of delivering the app to your device. For either Android or iOS, the best it can do is send a link via SMS. That doesn't work too well with tablets (unless you send the SMS to your Google Voice number, we suppose).
Frankly, the only use we can see for this --- the only use where it's more convenient than using the regular markets for either iOS or Android --- is if you wanted to see if an app was available in both markets, and compare them.
Want to see if Monopoly by EA is in both markets? Well, it is. It's $1.99 in the App Store and $4.99 (!) in the Android Market. It's rated 3.5 stars in the App Store and 3.0 stars in the Android Market.
Aside from that, since we can not just send a link, but the actual app itself to my device from the Android Market (and even have it start downloading and installing), Yahoo!'s app search is not really that useful.
Certainly, centralization in one tab is a nice feature, but it's not going to attract many users away from Google. After all, we can also use Google search and find App Store and Android Market links there, too.
Add Windows Phone, iPad results, and the ability to search multiple Android Markets, too, and you might have something there (though we'd still almost never go to Yahoo!, we'll be honest).