There are two things you need to consider:
1) Are there enough hours in the day for a single developer to do all that needs to be done - twice?
The 'default' dev environments for iOS and Android are substantially different, even down to the language (Objective-C on iOS, Java on Android), so it's not like you can write the code with a few macros and run it on both platforms. Essentially it's got to be written twice. That means you are going to have to either pick one for your dev to do first, OR you're going to have to wait twice (or more) as long for a simultaneous release.
2) Can you actually hire a dual-mode developer?
While lots of folks ARE qualified to do both, most are only up to date on one or the other. Can you actually find a good developer who has experience with both platforms in your area? My guess is that such people aren't nearly as common as good single mode developers.
If you can find such a person, you are going to have to pay more for them. If you have little enough work that they will have time for both sets of development, then they'll still probably cost less than two developers, but the minute that person runs out of bandwidth for multiple projects, you'll end up adding another person anyway.