Let's imagine that you decide to play at odds and bet $1 on every even number and $2 on every odd number. Total in the amount of your costs is $54. If you get an even number, your winnings will be $35 (1*35) (you lose $54-$35=$19). If you roll odd, the payout will be $70 (2*35) (you win $70-$54=$16). If it comes up zero, you lose all of that set – $54.
For me strategies should be simple. If you place a bet of 1x on even/red and a 2x bet on odd/black in my mind it would cancel out 1x from both sides its just psychological that you seem to be earning (as in get some money back) when losing.
However, for your example, if you place $1 on every even number and $2 on every odd number AND $2 on green you would pay $56. But it creates a loss-win-win situation. If even falls you'll lose $56-$35=$21, if odds or 0 you win $70-$56=$14. That $2 insurance on 0 effectively kills the total loss.
Now compare this with placing $0 on even, $1 on odd and $2 on 0. You pay $20 dollars. if you win with odd $35-$20=$15 if it hits even you lose and you pay $20. if it hits 0 you'll get $70. Why people don't bet on 0 is beyond me.
I believe the situation is different if you bet with 50/50 probability on both. Let's talk about betting on red and black. Say, you think blacks are going to win and you put 10 dollars on black. At the same time you put 5 dollars on red just in case. You spend 15 dollars. In case black is the winner, you get $20 or $5 of pure profit. I case red wins, you get 10 out of initial 15 dollars back. So, you either win or lose 5 dollars. If you only put 10 dollars on black, you lose them when the outcome is red, but win 10 more if the opposite case. The risk is higher with one bet and I kind of makes some sense to put money on both to lose less. There're some roulette strategies, but I believe that eventually you come to this rare case when something keeps going wrong and you lose everything, just like with martingale.
Same goes for this 50/50 bet. If you put $10 on black and $5 on red you should not compare that to $10 on black and $0 on red. It is the same as $10 (black) - $5 (red) = $5 on black. it is just the psychological effect that having $20 payed out (while spending $15) seems more profitable then having $10 payed out (while spending just $5). The end result is the same, you win or lose $5