You have had issues with both Decca and Suki.
A bigger two-in-one cage will not completely solve the issue, not when the budgies are determined and set in their ways.
The chasing and bullying will likely continue and if you happen to leave them unsupervised, one day you may come home to a severely injured budgie.
RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.