The Newman College students said they were surprised to win gold in the Multiple Event, where athletes perform synchronised routines in groups of two to five.
Elissa, of Woodlands, said she did not feel that she gave her best performance but was happy she won.
"I found it very easy (to synchronise) as my partner was the same age and build as me, so we could easily move the same way," Elissa said.
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Kaila, of Stirling, said she was proud of her performance but did have doubts going into the competition about synchronising with her partner.
"I found it quite difficult as my partner was younger and smaller, so the strength in our bodies was quite different," Kaila said.
"We had to train four times a week."
Both girls have been involved with gymnastics for years at Northern Districts Gymnastic Club.
A small group of West Australians competed at the September championships, with WA finishing in the top six in 15 of their events.
The group brought home seven gold medals, three silver medals and one bronze medal.