Cat Peeing All Over the House?

I have two 13 year old cats, a male and a female who despite living together for ten years, hate each other. Both cats have had UTIs in the past with the occasional spraying outside of the litter tray, but were put on special diets (which they are still on) to stop the UTIs which stopped the occasional spraying.

Over the past few months though, my female cat has been spraying/peeing more and more frequently so that now it's pretty much daily on things like the toaster, the kitchen bench, the couch, books, anywhere and it seems like everywhere! I recently caught my male cat spraying in the same place after I'd cleaned up after her so I am not sure how much of it is him, but I suspect it is her 90% of the time.

Her behaviour has been changing too, for example she used to be outside most of the time, but now rarely goes out, she has become quite clingy and she vocalises as though she's lost sometimes. The vet says she seems quite healthy for an old cat though and after finding it wasn't the UTI, recommended I try a Feliway diffuser. It's been two weeks and apart from the first two days, it doesn't seem to have made a difference, she even sprayed the wall right next to the diffuser! I have 3 litter trays that I scoop as soon as I see anything, but she/they pee on it or in front of it as much as in it!

I really don't know what else to do and am about to lose my marbles. Any advice, ideas, or similar experiences would be greatly appreciated. Please help!

By Emma S. from Melbourne, Australia