Without further information it is very difficult for me to provide the best advice but I shall certainly try .
Please reply when you can and we can continue the conversation.
Lethargy, a reduced appetite and diarrhoea can have many causes including an infection, toxin ingestion, dietary indiscretion (eating something they shouldn't have) etc.
As both are infected, it is likely to be something infectious or a result of something they both ate which they shouldn't have.
Exposure to a cleaning product can make cats unwell and cause e.g. oral ulcers and pain.
I would also double check for any other potential toxins such as a lily plant, human medicine or an essential oil they may have gotten to.
It is important they both have pink and wet gums, they don't have a bloated abdomen and their breathing isn't laboured.
Offer a bland diet of chicken and rice and plenty of water.
A vet visit is best if signs persist so they can be checked over. The vet may also advise on some tests such as a blood test or abdominal scan.
Treatment will depend on what is going on and may include e.g antibiotics and anti inflammatories.