Luis Veiga said:
If both options are available:
First and foremost what you bet should be based on your playing bank and how much risk you are willing to take. Then you make your bet sizes according to the size of the EV edge you have, or the EV edge the house has, for ONLY that next hand to be played. Based on the up front house edge, and the previous cards that have already been played in the current shuffle. When the hand is completed you now base the next bet on what the ever changing advantage or disavantage is.
Betting two hands of $75 will have the same risk of ruin as betting one hand of $100
Luis Veiga said:
Should I always choose to play two hands?
Not if you are the only player at the table, unless you know that the next round will cause a shuffle and that you want to get more money on the table before it is over.
Someone who always plays two hands throuout the shoe in a play all situation will need a larger spread to make up for all the negative EV hands that he/she are playing against. I do not recommend that type of approach as you would be adding too much volitility. I recommend you learn about different Wonging styles.