其实按你说的意思,前后两句都用过去进行时也是没有问题的
比如when the boys were playing basketball on the playground,the girls were playing volleyball.
但是按你说的非要一句用一般过去一句用过去进行的话,那就是when后面跟着的用过去进行时,比如：when he was stepping into the classroom,he found nobody was there.当他走进教室时,没人在那儿.强调的是走进教室这个动作.