Is it too late to rethink our child spacing decision?

I would be fine if I were raising each of my kids individually. My discipline techniques -- scolding and redirection for the 18 month old and time outs and consequences for the 3 1/2 year old -- work reasonably well in a bubble. But with the two kids together in one house, things are just not working out so well.
Even I can see the inequity -- when Julia does something wrong, she loses a privilege or gets sent to her room. And when Evan does the exact same thing, I tell him no and offer to read him a book. If I were Julia, I'd be pissed, too. But I'll be damned if I can figure out a better system.

The "let's be good role models and teach Evan" line gets parroted beautifully but is useless in practice. Giving Evan timeouts and taking away toys he cares about makes Julia happier, perhaps, but does bubkus in terms of preventing or correcting Evan's negative behavior. And Julia? Has taken up flinging her food off the table, emptying dresser drawers in giant heaps on the floor and other charming 18 month old habits. And then throws a temper tantrum worthy of a 2 year old when she gets a 3 1/2 year old's punishment.

Oh, Dr. Spock, where art thou? Calgon, you'd do, too...