Beating Law/kid wouldn't boost Teach and Shanks bounties that much
That's why Oda is waiting for Teach to beat Shanks to boost him to 5B+
Whenever someone with high bounty defeats an opponent with lesser bounty, they recited less than 500M increase
Look at Luffy ...he didn't even get 200M increase after beating Doffy. Even if you say he did it with law, both got barely 100M increase each. So overall, Luffy was gonna get only 200M increase if he did solo , all because Doffy's frozen bounty is below Luffy's 400M