As far as I am aware, the dative and accusative endings are always the same; they are the same as the nominative unless it’s a weak noun when they match the genitive unless it’s one of the weak nouns with the -ns genitive endings, but they remain the same as each other. (yay for grammar!)
It is a convention that the first ending is genitive and the second ending is plural, just usually the genitive endings are so regular and predictable that it’s not necessary to include them in the course.
If you go to a German dictionary, that’s the standard format they will follow.
Opening a random page in my dictionary, the first three entries are
Peitsche < - , -n > f
Pekinese < -n, -n > m
Pelikan < -s, -e > m
That tells us that die Peitsche becomes der Peitsche in the singular genitive and die Peitschen in the plural. So, in our memrise course we could write die Peitsche; - , -n, but that no ending is perfectly regular for a feminine noun, so it seems extraneous to include it. The same with der Pelikan; it is perfectly regular for it to be des Pelikans in the genitive singular.
But der Pekinese becomes des Pekinesen and that’s pretty unusual, and a pretty important grammar point, so it’s worth including in our format for the course.
You can argue that if it’s masculine and ends in an -e and is not Käse then it’s weak and people should just remember that, but I feel it’s not really that demanding to type der Pekinese, -n, -n instead of der Pekinese, -n. It’s a really succint way to include a reminder of an important grammar point. It could even be in parenthesis if people really don’t want to type it in.
There are also those even more irregular weak nouns like Buchstabe, Name, etc. with the -ns genitive ending and I think it is important to include those. It may be my own personal bias here, but I found it very useful when going through one of these 5000 courses to be reminded of the -ns irregularity.
I think it would be wrong not to include it, especially since it can be done so easily. If it’s not something you particularly want to do, then I’ll volunteer as tribute.