As a legal admin, you will be able to perform this in-app. The following will need to be done;
1. Cancel the original award as of its issue date to stop expensing and return the options to the pool.
2. You can then issue two new A and B grants. For example, if the original award were ES-1 then label the new awards ES-1A and ES-1B. All details from the original grant should be copied, such as board approval date, issue date, vesting schedule, and vesting commencement date. You can also attach any additional documents you would like. I recommend using Drafts v2 for this as it will make it much faster.
There is no way to link two accounts - each will need to be terminated separately.