Can I split the total claim amount for the child disability tax amount on line 318? The field auto fills the total claim amount and I can not adjust it.

You will not be able to adjust the amount of the Disability Tax Credit (DTC) transfer from a dependant using TurboTax.

This is how CRA says to file your tax return if you choose to split this credit;

You can split the unused part of this amount with another supporting person. If that is the case, attach a note to your paper return including the name and social insurance number of the other person who is claiming this amount. The total claimed for that dependant cannot be more than the maximum amount allowed for that dependant.