Raising siblings can be double trouble, but it can also be twice the fun. Where one may find something to chew on, two will play tug with it and rip it to shreds. But they will make up for it with all their cute cuddliness and entertaining antics. I'm speaking with the voice of experience here, I've raised siblings several times over (not a breeder, have never had a litter and all my dogs are spayed/neutered - I just seem to keep adopting them in two's
) Years ago I raised 2 collie half-sisters that were just days apart in age, then later brother/sister great danes and when they were just 6 months also got the collie that I now still have (talk about a triple
More recently I've raised a pup or multiple pups every year for the past 4 years - Lightning (BC/lab), Flash & Thunder (mixbreed rescue pups), Rain & Storm (BC), Flurry (BC) and Dru (BC)
Whenever I'm raising 2 together I always start them off sharing a crate, until they get big enough to need more space and then they each have their own crate (or even if they could still fit together I make sure they have separate sleeping quarters by the time they go for their spay/neuter surgeries).
Potty training can be a bit more difficult than with one pup, the easiest thing when they're tiny seems to be be to just scoop them up, one under each arm and carry them out. Otherwise while you try to get one out the door the other one is having an accident. Later, when they're bigger and used to following you better you should be able to just say something like "let's go outside" and encourage them to both follow you out the door.
My personal situation is such that I don't need
to do any leash walking so I don't start working on that right away. I have a small farm and a fenced backyard with lots of space. I tend to work on getting the pups to follow me and work on their recall without leashes, just by making it fun and making them want to stick with me. I want them to grow up knowing how to be off leash around the farm with me when I'm out doing chores etc. , plus with the work some of them now do as goose dogs they have to be able to work off leash in public. Having older dogs that already stick close to me does help I'm sure, pups always want to stay with me and the pack. I do of course teach them to wear a collar and leash at some point, but when I introduce the leash I do it with one pup at a time and at first I just take the pup out exploring around a bit and I follow them, keeping the leash loose. When it's loose they usually ignore it and I start to be the one to choose where we go and since I've already worked on having the pups follow me off leash there are generally very few conflicts. In your situation, you could work with each pup one-on-one for leash training, or since there is both yourself and your fiance you could each take a pup (but keep enough space between you that the pups can't be tangling up and playing with each other when on leash). It would be helpful if you teach the pups to focus on their handler rather than on each other. At this stage treats can be your best friend.
Lots of people teach their dog the command "watch me", rewarding the dog for looking up and waiting for what command will come next. With the breeds in your pups they should be intelligent dogs that are eager to please.
Ask 10 people the same questions and you'll probably get at least 10 different responses, but this is a little of what I've learned from raising multiple puppies. Oh yeah, besides letting them share one crate I also let them do pretty much everything else together as well, except if I'm working on something very specific (i.e. one-on-one training for something, whether it be agility, sheep herding or just some basic stuff like sit, down etc. though my whole pack will calmly sit in a circle around me when they know it's time for treats and everyone politely waits their turn) All this "togetherness" has never given rise to any problems for me, I currently have 8 dogs that are all able to live together and share the same space both in the house and out in the yard. The only one that gets separated at all is the youngest, who at not quite 9 months has not yet graduated out of his crate to having the privilege of full run of the house if I'm not home. The other 7 can be safely left loose in the house.