Create one account.
You can change the beneficiary of the plan (even to nephews, nieces, yourself or your wife) as many times as you need so long as you are spending the money on valid educational expenses.
Are you 100% sure both of your kids are going to college? If you aren't really 100% sure, a single account that you can move between them is the best bet.
Also, having recently looked in 529 plans, here are some things you have probably already thought about.
- Have you fully funded and done the most for your retirement? Don't pay for school until you are set in retirement.
- Research which states have the best plans. Only buy your state if it is a great plan, otherwise buy a good plan.
- Buy the plan direct from the state, not through your broker. No sense paying fees for it.
Look up good 529 plans here:
I don't think you can worry about fairly dividing the money up. I can see your wanting to be fair but what is more important, school or fairly dividing the money?
A 529 is money only for school. Assuming your kids aren't the same age and won't go to the same school, their expenses will likely be different. The younger kid will benefit from more interest from a longer investment, but suffer from having higher costs.
So if you want to insure both kids got $50K (for example) from you by the time it is all said and done, I think you would have to make that up from your own pocket. If only one child goes to school, any money you give the other for starting their own business couldn't come from the 529 without big tax penalties.
Depending on your position and finances you could state something like "I will cover your college expenses up to $50K" and then that is that. Just monitor your 529 and shoot for having $100K in the account by the time they are both college age. That runs a risk though, because if one child doesn't go to school your money is locked up for a while or will have tax issues.