How do I draw random frames that matches?

I'm trying to make a memory card match game for my child where matching 2 cards are not identical.

To be specific, the cards are 52 in total with big and small alphabet letters.

A matches small a, B matches small b and so on.

the cards are in each frame in one animation(A-Z, a-z) and all I need to do is draw out random frame number.

But I have no idea how to draw out random sets of (big letter, small letter).

Can anyone please help?

Always wanted to make one of these :)

dropbox.com/s/fg8aegqmpu91uid/MatchLetters.c3p

Always wanted to make one of these :)

https://www.dropbox.com/s/fg8aegqmpu91uid/MatchLetters.c3p?dl=0

I tried to edit the file you uploaded last time and try to make it myself, but it was too difficult.

Thank you so much. You're so kind. :')