Firstly no no no cooled boiled and add powder. The powder absolutely has to go into hot water as it isn't sterile and could contaminated with salmonella or enterobacter.
Frak's quick guide to FF:
Making fresh is best. A bottle will keep for around an hour at room temp and if you need to stop to change baby recap the bottle to prevent nasties from around and about settling on the teat.
Use 70C water to make up formula to kill the nasties.
Method 1 for immediate use or storing in the fridge for approx 12 hours
Boil 1 litre of fresh water and leave to cool for 30 mins
Measure out desired quantity of hot water
Add formula powder
Shake
Chill either using a tray of cold water with ice cubes/freezer packs or under running cold tap
Serve immediately or refrigerate for 12 hours (24 max)
Method 2 for screaming baby
Boil kettle with fresh water
Pour half desired amount into sterilised bottle
Wait 5 mins per 100ml so water reaches approx 70C
Add full amount of formula
Shake until fully dissolved
Add half required amount of pre-boiled cooled water so you've added the full amount of water and shake. The bottle will appear more full but that's because powder adds volume. It should now be room temp.
Method 3 for out and about
Pour 1l water into thermos flask
Pre-measure formula into little pot
Prepare as many bottles of half the amount of boiled/cooled water as you think you need with storage caps
Prepare as many sterile bottles with teats as you think you need
Follow method 2 above using water from the flask and topping up with water from the storage capped bottles as bottles are needed. If in doubt err on the hot side.
Beg a bowl of ice cubes if in a cafe or similar, otherwise have a flexible gel filled ice pack for rapid cooling
Alternatively if it's a short trip take premade bottles but use or discard within 3 hours if stored in a coolbag with freezer blocks, 1 hour if at room temp.
Tandem carrying is definitely possible or wrap one and have one in arms 