Start double manning tomorrow and not sure of tacho rules.
I will be driver 2 on a multi drop job. We will start unloading the truck about 30 mins after we set off and every 20 to 30 mins after that.
After driver 1 has done his four and a half hours of driving will I be able to drive straight away for my four and a half hours? or will I need to take a break after 6 hours from when we both started?
Tachograph might correct me here but
1 - You will need a 15 minute break before you can start driving
2 - If you have been a passenger for over 15 mins and your card is on POA or break then that counts I THINK as your 15 min WTD break. Ive heard it does count but there are countless arguments on this forum as to whether it does or not
3 - If the other driver does 6 hours work BEFORE his 4 1/2 driving is up then he needs a 15 minute minimum break,and unless you’ve had yours then you cant drive
Hope this helps
you can start driving straight away, it is assumed that during the 4.5 hours you could have taken a break. not rest only break as truck is moving, you do not have to wait to the 4.5 to change make it easy on yourselves swap after 3 hours or so. when you swap and take your breaks should not interfere with the days proceedings, I doubled for years far better to change more frequent less stress and keeps tired eyes at bay.
You can start driving straight away based on the timings you have given.
While you’re card is in slot 2 you’ll be clocking POA so you won’t be anywhere near the 6 hours work limit for the WTD by the time you come to take over. Plus the first 45 minutes of POA, provided the minimum is 15 minutes, will be assumed to be break for the card in slot 2.
You don’t need to wait until your mate has done the full 4.5 hours driving, you can swap over at any point and keep switching when it suits you.
Your tacho mode in slot 2 can only be on POA while the vehicle is moving. It defaults to it just as card 1 defaults to driving. Just be aware of the driving time. The normal breaks still apply i.e you need a full 45 minutes of continuous time in the passenger seat or 15 + 30 to clear your card. Best bet is to plan ahead and work out when to swap over depending on route.
If you want to drag it out then bear in mind that while you are not REQUIRED to stop for a break, you are ENTITLED to take a break. Your choice 
Thanks lads.
just remember if your in slot 2 make sure its on work when out the cab working