ManaRik
Why the T ?
Mana is まな, Rikka is りっか.
Japanese couple naming generally takes the first two characters from each name, so まなりっ.
Rikka’s second character is a small tsu that only doubles the following consonant and isn’t pronounced by itself, but like this it ends up being ManaRitsu. Shorting each character to the first Latin letter results in MNRT.