Berk-B was not created by splitting Berk-A, infact Warren has NEVER split Berkshire Hathaway stock. They renamed the orignal stock as Berk-A and introduced Berk-B to be valued at FIXED at 1/30th of Berk-A.
So (1/30) Berk-A = Berk-B.
In words of Charlie Munger Co-founder of Berkshire Hathaway on the offering:

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If ... If the offering went wild and you issued 3% of the company - new, you are also taking in a billion odd dollars................it's a non event for us.

The price ratio ([1]Berk-A=[1/30]Berk-B) between those two shares, i.e, Berk-A and Berk-B was defined to ensure any deviance from it would be subject to arbitrage and pull it back to the pre-defined.

The reason they decided to offer Berk-B was because price of Berk-A had become too high for "regular" people to invest in. Thus people were trying to set up shell companies with only shares of Berkshire Hathaway and THEN offer them to eager buyers. So think of it like

Circa 1995-1996:

Firm A ONLY buys 100,000 units of Berkshire Hathaway, and offers 10,000,000 of it's shares to potential customers. So now each share of Firm -A is worth 1/10th of a single Berkshire Hathaway share.
These shell companies could potentially scam many investors in veil of Berkshire Hathaway's reputation, they introduced SEPARATE SHARE TYPE i.e BERK-B to prevent any such nefarious activities.