After you file

So you filed as married filing separately and both of you claimed your child?  It is surprising that one of your returns was not rejected unless one of them was mailed instead of being e-filed.  Why did you file separate returns--usually the worst way to file?  You lose a lot of possible credits by filing MFS--you lose the childcare credit, earned income credit, and education credits, and you have a lower income on which to receive the refundable portion of CTC--the "additional child tax credit."

If you are going to amend your return, you could amend to file joint instead of just amending to remove the dependent from your MFS return, if you and your spouse can agree to file jointly.

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-married-filing-separately-returns-to-...

If you were legally married at the end of 2018 your filing choices were married filing jointly or married filing separately.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,000 (+$1300 for each spouse 65 or older)  You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.

If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI)

If  you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.

https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately

https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states

https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...