This method is actually a very unsafe
method of playing the roulette
table, considering that any possible loss will always be greater than any possible gain. Allow me to explain.
A roulette table has 36 numbers from one to thirty-six, and then two zeros on a standard North American table (Some European tables can be found with only one zero). Thus, if you bet on first dozen (1-12) and high (19-36), you will only be covering 78.9% of the board. While it is true that these are good odds and will end up in a technical "win" of sorts most of the time, remember that 50% of the board won't give you any "true win" at all, but nothing (i.e. zero gain, since the 'high' bet pays one to one, you get back the one bet you lost on the 'first dozen'). Thus, we can say that 50% of the board is eliminated. (The bottom 50% of numbers nineteen through thirty-six are actually your 'insurance policy' of sorts)
So, that leaves us with 1-18 and the two zeroes, essentially twenty numbers. That means, you will win 60% of the time on the first dozen, while losing 40% of the time on the rest (13 through 18 and the zeroes). But, there's an extra hitch. Whenever you lose, you're actually down two units instead of down one. Likewise, when winning, you're only up one unit. For example, if I place a $5 bet on 1-12 and an equal $5 bet on 19-36; a loss will yield a net loss of $10 (you lost both $5 bets), yet a win will only yield a win of $5 (you won $15, but $10 went to replenishing the bets). Thus, after you take in the weighting of wins and losses, you're really only winning 20% of the time, and losing 80% of the time, since you need to win twice in order to completely make up for a loss.
Remember that the first dozen pays out 2 to 1 while the nineteen through thirty-six will pay out 1 to 1. Using this scheme, you're only getting 0.5 to 1 every 40% of the time on the board-wide bets. Pretty crappy odds, if you ask me.
To make matters worse, both bets required are actually on the outside of the board, which requires higher minimum bets than on the inside. (Inside minimum bets are a fraction of outside minimum bets, in my case, $0.50 as opposed to $5.00 respectively). You're better off to play the table lotto style, where you place a chip on a select few numbers, and wait for the number to pay out 35 to 1. Your money will also last longer when you bet lotto style, as well.
So, the only one coming out ahead with this method of betting is the house.