pretend to be doing sth 的意思是“假装正在做某事”;be pretending to do sth 的意思是“正在假装做某事”。如:
He pretended (that) he was reading. 他装作在读书的样子。(= He pretended to be reading. ) (摘自《英汉多功能词典》新编升级版p.1319)
Were you just pretending to be interested ? 你只是在假装感兴趣吗?(摘自《剑桥高阶英汉双解词典》p.1724)
你的提问没有任何语境,所以两种表述都存在可能。