well. technically, the sheath is not a part of the weapon, it's like a part of the belt..
And those were not tantos, but smaller knives..
Various utility knives, awls, and even chopsticks were often mounted in various Asian (particularly Japanese) scabbards. Here is a reference to some:
http://www.geocities.com/alchemyst/glossary.htmI can't find a historical reference on a European sword with such things in the case, but considering the popularity of the practice across Asia, it seems perfectly logical.
As late as the early 20th century, Soviet-issue cav sabers had the bayonet for their old Mosin-Nagant 1891 rifles hung by a couple of clips on the outside of the sword scabbard. Can't find a picture of one at the moment, but there are buckets of the old things floating around Chinese antique markets (they're as worthless as old WW2 tank parts).
So it's a fairly common practice to attach whatever to your scabbard.