My new spouse already and mistakenly electronically filed a 2019 tax return (not with Turbo Tax), can I file a new amended 2019 tax return with Turbo Tax? If so, How?

Since your spouse's return has already been filed and is being processed, it is her return that will have to be amended to a joint return.  So use whatever tax software she used for that MFS return to prepare a Form 1040X that changes it to a joint return.    

If you did not use TurboTax to prepare your 2019 return, it is possible to amend the return using TurboTax but it will really be a pain.  It would be better to use the software or service used to file originally.  If a paid preparer made a mistake on your tax return, that tax preparer should do the amended return for you at no charge.

In order to use TurboTax to amend your 2019return, you need to purchase the desktop software for 2019 and re-create the return EXACTLY as it appeared when it was filed with the IRS/states.  Every line has to be the same.  Then you have to "trick" the software into thinking you filed by saying you will mail the return. (but do not mail it)   AFTER that, you use the software to prepare a Form 1040X, which has to be printed, signed and mailed to the IRS.  It takes the IRS about 3-4 months to process an amended return.

If you want to try this using TurboTax ---get the desktop software  (CD/download)  which has Forms mode.  That way you can see the forms and have direct access to them (you do not have that with online TT)---it would be easier to re-create a tax return with access to Forms.