Distribute assignments evenly among employees based on varied weighting of assignments...?

Hi - first time posting. sorry if the title is confusing.

I am trying to use Excel to help me come up with an assignment scheme for my employees - for when we have people on vacation.

Briefly, my employees utilize a "team worklist" which is loaded with individual requests from clients. We have distributed assignments based on the first letter of the client's last name. On a regular workday, when nobody is on vacation, each employee has between 3 and 5 letters of the alphabet that they cover. I have set up this distribution based on our book of business and the distribution of client names over the alphabet, and it works out so that everybody has an equal work distribution.

The issue I'm trying to puzzle out is setting up coverage schemes when two people are off. (When we have one person off, it has worked to just have everyone cover that person's letters, without specific assignments). Since each client's letter has a different percentage representation in the book of business, it can get a little tricky to distribute things out. My employees are very attentive to "fairness of distribution" so I need to be as equitable as possible with the distribution.

I have attached a file which shows the distribution of letter assignments across the team. Here is an example of a coverage scheme when both Patty (who is assigned M E V Y) and Kate (who is assigned K L P) are off:

Name Letters %
Jerry Split M (10%) with Walt 5.0
Walt Split M (10%) with Jerry 5.0
Cindy E L 6.4
Steven P V 6.3
Linda K Y 5.8

Based on 7 employees, there are 21 different permutations of "two people off". Thus far I've been coming up with coverage schemes on an as-needed basis, and basically sitting with a notepad and my spreadsheet and puzzling out combinations that work. Right now I have about 6 permutations puzzled out

So...my question is - can I harness the power of Excel to do the work for me?