Obtainable in BW but not BW2: Geodude, Ponyta, Exeggcute, Cubone, Hitmonlee, Kangaskhan, Jynx, Stantler, Smeargle, Chimecho, Kricketot, Cherubi, Landorus
Obtainable in BW2 but not BW: Meowth, Growlithe, Grimer, Drowzee, Koffing, Eevee, Natu, Aipom, Girafarig, Snubbull, Teddiursa, Skarmory, Phanpy, Makuhita, Nosepass, Meditite, Electrike, Gulpin, Spinda, Cacnea, Castform, Kecleon, Regirock, Regice, Registeel, Pachirisu, Shellos, Chatot, Spiritomb, Skorupi, Carninvine, Uxie, Mesprit, Azelf, Heatran, Regigigas, Cresselia, Zorua
Obtainable in one of BW but not BW2: Pidgey, Oddish, Paras, Machop, Bellsprout, Gastly, Porygon, Misdreavus, Houndour, Poochyena, Wurmple, Whismur, Starly, Shinx, Gible, Tornadus, Thundurus
Obtainable in one of BW2 but not BW: Mr. Mime, Sudowoodo, Skitty, Numel, Spoink, Latias, Latios, Buneary, Glameow, Stunky
Obtainable in one of BW and one of BW2: Caterpie, Weedle, Electabuzz, Magmar, Plusle, Minun, Gothia, Solosis, Rufflet, Vullaby, Reshiram, Zekrom
Obtainable in both BW but one of BW2: Ledyba, Spinarak
Obtainable in one of BW but both BW2: NidoranF, NidoranM, Abra, Magnemite, Rhyhorn, Chansey, Togepi, Mareep, Marill, Hoppip, Wooper, Murkrow, Lotad, Seedot, Ralts, Surskit, Shroomish, Slakoth, Aron, Roselia, Trapinch, Corphish, Bagon
All other Pokemon are obtainable in all Generation 5 games or none of them.
Note that my source uses a definition of "obtainable" that's kind of controversial. For example, Regigigas is obtainable in BW2 only if the player trades keys with the opposite version. So it's similar to the version exclusives because it requires connecting with another game to obtain, but my source counts Regigigas as "obtainable". I don't know how many other Pokemon are like this.