Im not sure if you mean for the regular Johto Pokedex or the National Pokedex, so I'll do both.
For the Johto Pokedex, if you have just HeartGold and SoulSilver, you'll be fine, as every single Pokemon in the Johto Pokedex can be found in those games. The exceptions are the Johto starters, of which you can only get 2 of the 3 if you play through both games once, but this can be avoided by restarting one of the games, getting the final starter, and then trading it over to the other, so you're fine for that one.
As for the National Dex, though, that's a different story. In addition to both HG and SS, you will need a copy of both Diamond and Pearl, and either Ruby, Sapphire, or Emerald; any of those will do. You will need both Diamond and Pearl to obtain the three Sinnoh starters, both Sinnoh fossil families, the Drifloon family, the Stunky family, the Glameow family, and all the Sinnoh legendaries barring Regigigas. The copy of R/S/E is for obtaining the three Regis, and by extension, Regigigas. And then there's the matter of the Kanto and Hoenn starters, which have the same problem as the Johto starters, but they can be obtained in the same way as them as well.
Anyway, to recap, the Johto Dex isn't at all hard to complete provided you have both the Johto remakes, but the National Dex, while it is possible, is really freakin hard.
Hope I could help!
