I'm of the two are better than one school. But, make sure that there's a reasonable difference in age; with a big breed like a Leonberger, I'd say 6 months. I have had two Irish Wolfhounds that were 3 months apart and they were five times the trouble of two adults; always up to mischief, and irresponsible, they destroyed a 3 piece suite when I left them with an adult dog for only quite a short while (when I came back, the adult had a look which said "Don't blame me. I couldn't stop them!"), and that was typical, as was their contriving to break two panes of glass in the French doors and take a panel out of a door! One large pup can do that, but two together is one too many.
And I'd have two of the same sex. With two dogs there's not usually a problem with dominance, given the age gap; sometimes the younger one tries it on with the older when the younger one reaches late adolescence, but that passes. Two bitches, unspayed, tend to synchronise their cycles and they come into season together, so that's one less problem.