There is not a straightforward and simple way to do that.
Creating a Married Filing Separately return for one of you in an online account that you will not file would give an estimate of how the refund might be split. However, if certain credits are involved, then the answer will not be an accurate one since there are some restrictions with claiming credits when using Married Filing Separately.
If you are using the CD/downloaded version of TurboTax, there is a “what-if” worksheet that can be used to compare Married Filing Joint and Married Filing Separately, but again the answers would not be exactly what you are looking for.
One thing that you may consider doing is to file an Injured Spouse form with your return. The purpose of this form is to protect your share of the refund on a joint return when one spouse has a previous debt that would seize a tax refund. It will take longer for the IRS to process the tax return, but only the refund attributed to the person who owes the debt would be used to pay the debt. The Injured Spouse form itself will allocate the income and deduction items between the two parties, but it does not fully show the division of the refund because certain credits must be allocated by the IRS when they process the return.
See the information below for more details about the Injured Spouse form as well as the “what-if” worksheet.
https://ttlc.intuit.com/replies/3326788
https://ttlc.intuit.com/replies/4775330