Can't enter a stock spin-off as a split in a transaction

[rhill]

Here is the problem I have:

Let's say Stock A spin-off Stock B on a specific date. I am trying to
enter the following transaction (with made-up numbers/date) to record
the spin-off:

Transaction:
1901-01-01

Split 1:
  Action: RoC [as in Return of Capital]
  Memo: Stock A spin-off: new ACB of Stock A [adjusted cost base] = 60%
of ACB of Stock A before spin-off
  Account: Stock A
  Share: [empty]
  Price: [empty]
  Buy: [empty]
  Sell: $6000

Above worked fine. Now:

Split 2:
  Action: Add
  Memo: Stock B issued by Stock A: ACB of Stock B = 40% of ACB of Stock
A before spin-off
  Account: Stock B
  Share: 50
  Price: [empty]
  Buy: $6000
  Sell: [empty]

After I try to record the transaction, I am asked which field should be
calculated by GnuCash regarding the split for Stock B, to which I say
"Price". Then GnuCash proceeds to change the field "Share", setting it
exactly to the value 6000. I can't find no way of preventing GnuCash
from overwriting the "Share" field with the amount appearing in the
"Buy" field, while it sets the "Price" field to "1". I tried all sort of
ways, changing the order, etc, result is always the same, even when I
explicitly enter the "Price" of Stock B, it will revert it back to "1"...

Essentially here, it is to reduce the value of an asset, Stock A, while
increasing the value of another asset Stock B, but GnuCash does not go
along.

I don't know how to solve the above, that is without creating two
distinct transactions, which involves creating a fake disbursement in
some unrelated third account.

Any help appreciated.