Can I file my 2020 return while my 2019 return is still processing?

You can go ahead and file 2020.   Did you use the recovery rebate credit for the stimulus?

The stimulus check is an advance on a credit you can receive on your 2020 tax return.  If something went wrong or you did not get the stimulus check in 2020, you can get it when you file your 2020 return in early 2021

The Recovery Rebate Credit will be found in the FEDERAL REVIEW section. ( you should see "Let's make sure you got the right stimulus amount”)  If you are eligible it will end up on line 30 of your 2020 Form 1040.

Make sure that you enter ALL of the stimulus money received for the 1st ad 2nd stimulus for yourself, your spouse and your children.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-2020-tax-return

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.

Yes you could file, however if possible I would wait to file my 2020 return after I have received notice the 2019 return has been accepted.  This may help with duplicate processing issues. 

I would not hold your current return past the required filing date for your 2020 return. You want to make sure the 2020 return is filed on time. 

Last year, the deadline for filing your federal income tax return was pushed back from April 15 to July 15 because of the COVID-19 pandemic. However, even though the start of tax season is later than usual this year (February 12), the April 15, 2021, due date hasn't been changed – at least so far.