。
Who speaks English better, Yoko or Yumi?
Literal
Yoko-san [and-と] Yumi-san [and-possessive-との] [at-では] which [subject-が] English [object-を] well speak-can-[question-か]?
AとBとでは is the comparative structure 'between A and B.' どちらが ('which [of two]') is used for comparisons. 上手に ('well / skillfully') is the adverbial of 上手 (na-adjective 'skilled'). 話せる is the potential of 話す.