Is the singer using a PA mic? If so, can you get a split feed from it.
Depending on how "live room" vs how dry they want the recording, an overhead shot covering
both the piano and bass about 4-6 feet out or for a dryer sound, can you get a good dynamic
for the bass at about the bridge and a separate mic looking under the hood of the piano, assuming
a grand piano. If you can get to a warm up / practice prior to the performance, experiment and
listen, listen, listen.