It was actually a really interesting development process. We already had the web app, and our main UI developer is an Apple guy, so he took on the iOS version. Meanwhile I'm primarily a Linux guy and work on FastMail's backend services, but also an Android user, so I did the Android version.

What it meant is that the other guy could make supporting UI tweaks as he needed, and I could make supporting backend tweaks as needed (eg push system integration), but we had to come together a lot to find the similarities and differences between the two platforms and do changes for each other.

It became a bit of a competition and we both were working very fast. We had something we could use every day in less than a month and we each learned a huge amount about the other person's area of speciality.

I'm fairly confident if we did just one app first we would have gone much slower and made some bad architectural decisions that would have hurt when we got onto the next platform. If we ever do a third platform client it'll be really easy.