A Tower Controller reported a NMAC between two aircraft at the same point on final approach 200 feet apart with the higher aircraft descending.

Narrative

I was in the tower cab; had just been relieved from position maybe 10 minutes before I observed a coyote on runway XX right and alerted the tower of the coyote. There was an aircraft rolling out on XX right at the time and the local controller issued the coyote traffic. There were other aircraft on final to runway XXR so the OS (Operations Supervisor) and I were watching the coyote to make sure it didn't go back on the runway. About 30 seconds later we looked left which was short final and I noticed two aircraft over the runway XXR display threshold and at first I thought the aircraft on top had to be for runway XXL; but the size of the aircraft and my experience was telling me that it was actually directly above the other aircraft who was also landing runway XXR. Just as I comprehended what I was seeing the controller working LC2 (Local Controller 2) must have seen the same thing and issued a go around to the aircraft above the other aircraft. My guess is their closest proximity was less than 50 feet. Neither aircraft reported seeing the other aircraft. After reviewing the falcon; it is clear that the two aircraft involved were in the traffic pattern and the sequence given was not followed by the pilot who inadvertently turned base directly into the aircraft they were to follow who was on final. At no point did either aircraft report the other in sight and for about a mile they were directly on top of each other; separated by 200 feet and decreasing. At the time of the event; Local Control was split and one OS was working the back and monitoring both frequencies. This is another example of needing a local assist position to help either the controller working or the OS/CIC (Controller in Charge) monitor assists position instead of making one person responsible for monitoring multiple control positions. A local assist would've been able to monitor either the coyote on the runway or assist the local controller and ensuring the aircraft he was sequencing were following his instructions.