@Patou88
Before I had two dogs, I only had one, who also had to deal with long days from 8.30am to 6.30pm on his own, right from when he was a puppy.
At the time, I couldn't find a pet sitter I trusted to drop in, and I didn't have the money to pay for a professional walker.
I never had any issues with him—no accidents in the house (mind you, until he was about 6 months old, he was left in the garden with access to a shed so it didn't mess with his house-training), no chewing things, and no barking while I was out (after the first week of getting him used to it).
I’m not saying it was the perfect situation (and I know the idea of a puppy being home alone from 8.30am to 6.30pm will make a lot of people's hair stand on end), but he seemed to cope with it just fine.
I'm not saying it would go that smoothly with every dog, and I’d actually planned some backup options in case he couldn’t handle it, so I wouldn't end up in a situation where I had to rehome him because of complaints about barking from the neighbours.
I think it’s more a matter of getting them used to it AND their individual temperament (a dog that's well-adjusted, without anxiety issues, etc.) rather than whether they're an adult or not, or whether they're totally alone or with another dog.
Back then, my dog likely spent almost the whole time I was gone asleep (I didn’t have a pet camera then, but since nothing in the house had been moved while I was out, it’s the most likely scenario), and now that I have two (with camera footage to prove it)... it’s still the case! :D
But I feel better about it now, because even if they do spend the day sleeping, at least they have the option to do something else if they want to.
The only real issue when a dog (who’s used to it) stays home all day like that is managing to meet all their needs (in terms of physical and mental exercise) in the few hours left before bed. It makes for very dog-centric evenings. Walks, playing, training, cuddles—not much time for anything else. By dedicating a huge chunk of my evening to them, even after long days of being inactive, they’ve always seemed happy to settle down at night.
Nowadays, I have a pet sitter who, except for the odd occasion (like when she’s on holiday), drops in at lunchtime when I'm working long hours. But I often wonder how much better it actually is for them, or if it’s just better for my own peace of mind. As it happens, the younger of my two dogs almost always causes a bit of mischief after the sitter has been (it’s like he’s venting something, an emotional overload or frustration that her visit was so short—only 30 mins—I don’t know), whereas he doesn’t do it if she doesn't come at all, even if it’s a long day. :/