Female Cat Spraying Inside?

I have an 18 month old desexed male inside cat and a 10 month old desexed female who also is not allowed outside. 3 months ago she has started spraying and weeing in all different parts of the house. I can be standing in the kitchen and she will just come in and wee in front of me. I have 2 litter trays which both cats use. Someone suggested that she may not like the odour of the male's urine in the tray, hence won't use it, but the trays are cleaned daily.

I know she is about to spray because she runs around like a maniac and then jumps up on the couch and sprays over the wall. I have bought the spray "Urine Off" and regularly spray where she has sprayed, but this is no deterrent, as it just doesn't work. I have taken her to the vet many times to eliminate a bladder infection, etc., but she is perfectly healthy. Unfortunately my next step is to give her "Valium" or a similar drug which I don't want to do. Any suggestions would be really appreciated as I just don't know what to do next.

By Sandra