Вчера, когда они переходили улицу, они встретили старую знакомую. - They ran into their old acquaintance when they were crossing the street. Вчера, когда они перешли улицу, они встретили старую знакомую. - They ran into their old acquaintance when they had crossed the street.

Thank you for your help!

Hi! Difference in the meaning. Like in English tenses: переходили here - past perfect continuous (means - they met her while they had been crossing the road yesterday). Перешли - past perfect simple (means - they met her when they had crossed the road).

Glad to hear it :) Thanks

doesn't matter absolutely. but if you want to attract attention on the process in past (переходили), if this process will be important for this conversation,. then use it. (перешли) it is just like a fact. i would use both of those variant.