I'm guessing it's syncing a seed to srand(), so neither of you control when the messages pop up

That's elegant.

So, OP, I have to know: did you come up with the synced seed first and then try to build an idea around it, or did the idea of simultaneous, server-less, auto-generated endearments drive you to this solution?

I had the gift idea for my girlfriend a few years ago. How to implement it came next (even so the tricky part was how to sync the seed)

Interesting. I really do like it. It's naff but in the best possible way. (I'm working on something related for my wife at the moment but it's even cheesier than Twosome - not sharing it here :])

Do you do some sort of seed exchange/negotiation? Something like swap current system times, and use the higher as the seed?

Something similar: both phones exchange a random seed and compute the arithmetic mean in order to get a common seed :)

This biases the chosen seeds towards the middle.. You could do something like add them both together modulo the size of whatever the random seed is on the platform, that should be unbiased.

That's correct. So once connected you will always get random notifications at the same time, even with NO network :)

Aha, that's what I was missing. I didn't realize from the app description that it was a randomized message, and I thought it was one device's user sending a message to the other.