They share a stat allocation because bonus stats are based on your class. Jobs are extensions of your class, not replacements for your class.

You're free to disagree with this, but that's the way the system works. Your choices are to favor one job, the other, or split your points accordingly. It may seem a little weird for arcanists, but the upside is that arcanists get an extra job for "free". Other classes will probably get a likewise deal in the future.

Some people will say that the stat allocation doesn't matter, but I strongly disagree. 30 points in a primary stat is an entire i90 chest or leg piece of gear. You don't see people refusing to wear certain piece of armor because "it's a hassle and I can beat it like this anyway". That sort of logic is nonsensical, and so is simply dismissing 30 points of stats in your main role. Still, if you like both jobs, you have to make a sacrifice somewhere. Perhaps try some different dps/healing classes?