Greetings, romad.
It appears that you're having a problem pairing your new Apple Watch Series 5 with your iPhone X. You receive a message that it couldn't pair with your iPhone because it’s on iOS 13.7. You also mention that your old Apple Watch is also paired with your phone and wondering if this is the reason. We understand how important it is to pair your new Apple Watch in order to use it, and we're glad to help with this.
Set up and pair your Apple Watch with iPhone - Apple Support -- If your Apple Watch has watchOS 7 installed, you'll need to update the iOS on your iPhone X to iOS 14 or later in order for it to pair correctly. After updating, see if you're able to pair your new Apple Watch with your phone.
It shouldn't matter that your old Apple Watch is already paired. You'll be able to switch to which watch you're wearing once the new watch has been added.
Feel free to get back to us with any questions or problems you may have.
Take care.