省略are不对吧

Thisrequires investing in technology; the few banks already selling custodysubcontract tasks to specialist firms.

我感觉selling前are不可以省。各位老师如何看？

这里的 selling custody 是修饰 banks 的目前分词短语，作后置定语。后面一个分句的谓语动词是 subcontract，涉及subcontract sthto sb（将某事转包给某人），即 subcontracttasks to specialist firms 指将业务转包给专业公司。