It has been established that bilinguals activate both languages even when only one language is being used. However, little is known about how the two languages are co-activated during simultaneous interpreting (SI), a demanding task involving intensive code-switching. This study investigated (1) the effect of task on cross-language co-activation and (2) the time course of co-activations triggered by form and meaning. Thirty-one professional interpreters were recruited to complete a cross-language task (English-to-Chinese SI) and a within-language task (English-to-English shadowing) with their eye movements tracked. Participants heard English passages which contained critical spoken words, each paired with a visual display of four Chinese words. One of the words was a competitor that resembled the translation equivalent of the spoken word in either form or meaning, and the other three were unrelated distractors. We found that participants directed more visual attention to both types of competitors at an early stage in shadowing, while the word-form competitor effect occurred during SI preceded that of the semantic competitor. Our findings support the parallel account of SI processing, with implications provided for the relationship between cross-language interactions and the time lag between input and output during interpreting.