It's in the Senate rules for impeachment: https://www.govinfo.gov/content/pkg/GPO-RIDDICK-1992/pdf/GPO-RIDDICK-1992-69.pdf They were last revised in 1986, but no one is going to televise FISA hearings on the same day as an impeachment hearing is going on.    Revising the Senate rules to allow them to act on the impeachment articles regardless whether they have been delivered from the House could work.