It looks like this thread is going heavily on gas management techniques...
However, as you are a SM newbie, and apparently beginning with OW dives, you should begin your practice at a much earlier stage- before you get to start diving, breathing and managing your gas between two tanks, you will need to actually rig the gear and put it on you.
It probably starts with how you are going to rout your hoses- most regulators have several HP and LP outlets to choose from. You'll have to experiment a little and find out which of these ports give you the best solution for each of your regulators. Most probably they are going to be different one from the other, so you'll have to start getting used to having a "left reg" and "right reg", and it will take a few trial and errors until you figure out which is the best hose routing for each.
Then, you are saying its also a boat diving vacation, so you need a solution for the times when you're on BM. Will you need to keep one of these regs also functional for BM, or you'll be bringing a third reg with you? If you have only two regs, one of them part time shared for BM, tt may have some impact on how you rout the hoses in one of these regs, and you're going to need some solutions to tuck the spare stuff (e.g. the octopus, SPG with relatively long hose etc) so it doesn't interfere with you too much during SM.
Then, you need to figure out how you bring all this stuff to water line, and put it on you in water. No, it is not rocket science, but then you discover that you need always to remember what to do with the LP inflator hose before and after the five (don't forget to disconnect it before you unclip your tanks
). You should practice a few times putting the tanks on, removing them before the dive and also unclipping and clipping them back (and perhaps also moving them to waist d-rings too?) during the dive. Again, no rocket science and probably after a dozen SM dives you won't need it anymore, but when newbie you need to build the "muscle memory" for these new movements. What about reaching pockets, or wherever you put extra stuff such as SMB etc? You may need to re-arrange the location of this type of extra stuff according to your experimentations.
Soon you'll need to inflate/deflate your wing. You'll have to find out how comfortable you are with it with your SM configuration, perhaps you'll also find the need to rout the inflator in a different manner?
You'll soon find out that there are more than one way to do things in SM...You just need to adjust to what suits you, and better do it in a "controlled environment": If you can have a buddy/friend who can snorkel above you while you get to 1-2m depth and see (or better photograph) how the gear sits on you, how balanced it is and how much clutter you have from your regs etc. You'll probably discover that you need a few more iterations until you get the right arrangement.
So, basically you need to practice what is probably considered "trivial" things, but you need it to gain confidence and acquaintance with a slightly different style of diving. It will take probably 2-3 iterations until you get the rig arranged in a comfortable manner, maybe more, and a few experimental dives until you begin figuring out what's the best way for you.
