There's no problem putting both of these devices on the same voltage rail, PROVIDED that you have enough power to keep them within spec. Most hard drives spike their power on spin up, but anything modern should also have a limiter that keeps that constrained. The hard drive you choose will matter a good bit, but just as a reference point I have a 1TB "standard" WD 3.5" HDD here in my desk that is labelled at 0.66A @5V and 0.55A @12V. There are definitely other drives that use more power, but I think you'll find that these values are pretty common for most "general purpose" drives that are out there today. If we add up the power rails assuming 0.5A for the Pi itself, we get:
5V rail: 0.5amp for the Pi + 0.55amp for the HDD -> 1.05amp
12V rail: 0.0 for the Pi + .66amp for the HDD -> 0.66amp
My personal experience is that most things like hard drives are conservatively labelled, because manufacturers want to re-use the same motors and other equipment and it's perfectly fine if they over-state the power on the label (whereas under-stating it gets you in trouble)... so given that you're only at 50% load on the 5V and barely over 50% on the 12V rail, I'm pretty comfortable saying you're OK with this.