So if we were to get one later than the other, what would be a good gap?
From my experience I'd at least eighteen months. You want the first dog to be past the puppy and adolescent stages before adding another. My youngest two have almost exactly twelve months between them and in hindsight that was far too small a gap, it was very hard work indeed at times.
How long roughly does it take to house train a puppy?
This can vary hugely between dogs, as an average I'd say they're generally starting to get fairly trustworthy by about six to eight months but it can be quicker (DWhippet3 had it totally cracked by five months) or much, much longer.
And what age does the chewing phase usually end?
Again, this really depends on the dog. Two of mine will still investigate a new piece of furniture by giving it a quick chomp if I'm not careful and they're three and four years old
On the other hand DDog2 has never been a big chewer, neither has DWippet1 although both had their moments as puppies.
One more question, how much extra cleaning does having a dog involve please?
Yet another "depends on the dog" answer
The more shedding then obviously the more hair there'll be to deal with. It also varies how mucky different dogs get on walks and how easy they are to clean so it doesn't end up all over your house. There's the cleaning you don't really think about beforehand either like the interminable nose prints on windows/screens/doors. It's inevitable there'll be bodily fluids occasionally as well, even fully house trained adults can get caught out when they're poorly. I've never had a house without a dog so can't really compare cleaning levels with not having one but I will say as far as I'm concerned the level of cleaning I have to do is worth it 