Ranger, I know your frustration (I guess many of us do). I'm no stranger to pee in the house. I even resorted to buying special flashlights that help me find pee. I've had to throw away my sofa. I've had to throw away some really nice cat cushions I got. I have to cover my bed with unsightly waterproof sheets. The boys are no longer allowed in the room that has my keyboard (synthesizer) because they've peed on it and the pee got down in between the keys.

I can't tell you how unpleasant it was for me to work on my synth during the summer months. (

) Don't get me started on pee in accordion doors with railing.

For us, the pee issues suddenly start up and then suddenly stop. Something must bug them or remind them to start marking areas, but I don't know what it is.
(I do want to mention that it's a little curious that your cats are now suddenly throwing up and having diarrhea - it might suggest they've been feeling off, or can smell the other is off?)
What I might suggest:
1) If there are cushions or blankets that have been peed on a bjillion times, you'll want to wash those (obviously)
really well. Then what I do is I put a pet sheet inside a pillow case and put the pillow case over the cushion so if there is any pee issue, the cushion is all right.
2) I'd cover the places that are expensive or difficult to clean with a waterproof sheet. You could wrap the bottoms of your furniture with the sheets (while you're out or sleeping) and make it an unpleasant/difficult place to pee. I have sets of waterproof sheets so that if I need to wash a sheet, I have a spare one to replace it while it's getting laundered. It might look awful but the idea is to make them not want to pee there anymore. My experience is that they do give up on the area. I would recommend waterproof sheets because they are so much easier to clean since they absorb the pee. I wouldn't, for example, use a plastic tarp because that will mean having to clean the tarp, tracking and leaks.
Something like this... It looks like they're called waterproof bed
pads:
https://smile.amazon.com/dp/B00ZZ00G0Y?aaxitk=gh9hu5n5jwwbv5z39MMLUQ&pd_rd_i=B00ZZ00G0Y&pf_rd_p=44fc3e0f-4b9e-4ed8-b33b-363a7257163d&hsa_cr_id=3304771950901&sb-ci-n=productDescription&sb-ci-v=Vive%20Washable%20Incontinence%20Bed%20Pad%20-%20Heavy%20Duty%2C%20Absorbent%20Waterproof%20Urinary%20Mattress%20Protector%20-%20Overnight%20Sheet%2C%20Reusable%20Dry%20Bed%20Wetting%20Urinary%20Underpad%20Mattress%20Protection%20-%20Elderly%20Seniors%2C%20KidHang in there! You're not alone! <3