I just paid my 2018 Fed and State return taxes due with Direct Debit but my first 2019 Estimated Fed Tax is due today as well, can that be paid online as well?

Yes, you can pay your estimated federal payments online. You can use the IRS direct pay website to pay your estimated tax due.

https://www.irs.gov/payments/direct-pay

Most states allow you to do the same. You will need to visit your state website to do so.