How to wirelessly back up two Macs to the same shared external drive using Time Machine?

I will not answer your question directly but it is not good practice to put all your backups on one drive. Should that one drive fail you have lost it all. I would use separate drives.

You could use a NAS (Network Attached Storage) such as Synology.com. There are other consumer NAS vendors.

You can get NAS boxes with 2 redundant drives, with typical 4 or 5 drive bays. 6 or more bays then to lean more towards a commercial unit (maybe small business). Many NAS boxes can have add-on expansion bays, which can increase the totally number of drives and of course the amount of storage.

Generally speaking a NAS unit sets up either mirrored drives (one drive is a 1-for-1 copy of the other), or RAID 5 (1 redundant drive for the other drives in the box) or RAID 6 (2 redundant drives for the other drives in the box).

Some vendors also offer other services for your Macs, iPhones, and iPads, such as picture servers, your own personal Dropbox, a shared notes server, etc...

To add to the good advise from @tbirdvet 👍

Backing up to Network Attached Storage (NAS)

For primary backups, we recommend that you procure a USB or Thunderbolt hard drive and create a backup on that locally-attached disk.

3-2-1 Rescue Plan

To truly protect your non replaceable Data

Have a 3-2-1 Rescue Plan in place and always current

3 Backups using 2 methods and 1 off site incase of natural disaster or un-natural disaster.

Each of the above should be done to a Dedicated Single Purposed External Drive directly connect to the computer(s)

Below link is intended to augment what TM Backup does 

https://bombich.com

Yes you can, but maybe not the best idea. I’ve done it several times as a transfer method but not for extended use.

A Time Machine volume is your bail out when something goes wrong on your computer. With two backups on the same drive you are doubling the amount of work the external drive is subjected too.