2019 Mac Pro Tower - Slots to use to insert 2 pci cards

OWC proves themselves on their support.

Their web site says it is appropriate for Mac Pro 2019, is an x4 card (so goes in any slot) does not require aux power, does not require Drivers, softRAID is included free but NOT required.

They also have an installation Video for this product.

The article I cited tells how to assign to A or B.

The bottom ones (slots 1 through 4) are x16 (widest, fastest data) for supporting MPX devices and high-end graphics cards.

As you go up, they are more appropriate for 'ordinary' cards. Most PCIe drives can go in any slot -- they do not need the fastest data transfers possible. The topmost slot has already been grabbed for Apple's half-width x4 I/O board.

You also need to think about aux power connectors, if needed.

Once installed you will have to assign your card to either A or B side of the bus using PCIe slot Utility. All described in this article:

Install PCIe cards in your Mac Pro (2019) - Apple Support

¿what are you installing?.

From what I can see on their web site, you can install both, or upgrade the drive "sticks".

Do you have adequate backup drives? for Time machine, these should be large and External (so that they do not share too much Hardware and die at the same time if your Mac dies). There is no additional benefit to drives that are fast.