+1 -- On the street, esp. just after dark, or where artificial light is plentiful, I run one flashing, because I definitely get more notice from the cars. Sometimes I run both flashing under these conditions, because I think the helmet light gets more notice because it is up higher, and it moves with my head

On the MUP, if there are other cyclists around, no flash, or only a very low powered flash except when coming to street crossings, perhaps.