How should I test for when 2 players have a total score of 20 when added together?

Answer

scoreboard players operation #sum myScoreboard = @a[tag=player1] myScoreboard
scoreboard players operation #sum myScoreboard += @a[tag=player2] myScoreboard
execute if score #sum myScoreboard matches 20 run say Player 1 and Player 2 have a total score of 20

Explanation

Line 1:

scoreboard players operation #sum myScoreboard = @a[tag=player1] myScoreboard

Line 1 copies the score of Player 1 to another container. To do this, a fake player, #sum, is used. The objective this fake player belongs to does not matter.

Using the scoreboard players operation command, #sum's score is set equal to Player 1's score using the = operator.

Player 1 is referenced by the target selector, @a[tag=player1]. Research target selectors for more information.

---

Line 2:

scoreboard players operation #sum myScoreboard += @a[tag=player2] myScoreboard

Line 2 adds the score of Player 2 to #sum. Similarly to line 1, scoreboard players operation is utilized, except this time, the += operator is used instead of =.

---

Line 3:

execute if score #sum myScoreboard matches 20 run say Player 1 and Player 2 have a total score of 20

Line 3 tests if the score of #sum matches 20. If it passes, "Player 1 and Player 2 have a total score of 20" will be displayed in chat.