I wouldn't consider that to be weak. You're betting more than you did the time before. Chances are decent that they'll call again if they called before and the flush didn't help them, unless they put you on the flush, which wouldn't really make you unhappy anyway, since you'd get the pot. I'd be more hesitant about something like this in a multiway pot, but with just 1/2 callers, I wouldn't consider it weak.

Another option though is what someone else mentioned doing: bet exactly the opposite from how you did it - half the pot on the flop and then if the flush doesn't come, the whole pot on the turn. Doesn't help when it does come (except that you lose less money...and it's hard to bet high enough for a flush draw not to be callable on the flop), but when it doesn't you're getting people to call for the odds of hitting by the river while only really giving them the chance to hit by the turn.

I know I didn't completely answer your question - hopefully someone else will. I figured the viewpoint would be worth considering though.

- Jeffrey