So, to clarify -
Squeaks had a UTI a few months ago. It was treated and cured with antibiotics, but the vet told you to give cytease long term. He had no further issues.
The other piggy had no issues but you were told to give a preventative of cytease anyway.
Neither piggy had any sign of issues a month ago but both have started squeaking while peeing a week ago.
A UTI is caused when bacteria from faeces gets into the urinary tract. Antibiotics (and painkillers) are given and the issue will be dealt with. No long term treatment should be needed.
IC affects the glucosamine layer in the bladder. It is a long term condition which can’t be cured but instead managed with glucosamine, painkillers and keeping a stable diet. It is thought to be caused by stress, and piggies born to stressed mothers tend to carry high levels of stress throughout their lives (nothing can be done about that).
Both cause pain when peeing and blood in the urine (although clear urine can test positive for blood).
If no bacteria is found and no other treatable condition is found but symptoms persist, then IC can be considered, diagnosed and treated accordingly. It is a long term battle with peaks and troughs, not something you can deal with quickly.
Perhaps you could try catching their painful pees on video (I know, won’t be easy), so you can show the vet how they are squeaking when peeing.