How to quickly split transaction between two accounts in GnuCash?

Here's something that I do frequently in GnuCash:

1. Import transactions for my checking account
2. Most transactions are automatically mapped to an expense account
3. For transactions that I agreed to split with someone, edit the transaction so the amount for the expense account is split in two and assign the remaining half to another account (leaving the amount withdrawn from my checking account unchanged)

How can I quickly assign half of an expense to another specific account like this?

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