Advice needed for cat food

Hi, I have 2 rescue kittens about 6 mos old.  One of them, vomits about every 2 days.  Usually it is liquid but sometimes it appears as though the food wasn't digested.  I am not sure which one is doing this as it always occurs when we are sleeping.  At first, I thought it was because they wouldn't be allowed to sleep with me or in the same room all the time but now I'm not sure what is causing the issue.

I feed only a tablespoon of wet food twice a day to the one that I thought had the issue (Shadow) but have since started doing it to the other cat as well (Yohan) as he eats his food quickly and then tries to eat her food.

I can't afford to keep switching food to try to find the best fit and our vet is on vacation next week plus I can't afford to have both of them tested to see which one isn't feeling well.

Any suggestions?

Thanks.

What are you feeding them now? 

Currently, they are on Natural Balance foods (both wet and dry).  It is they Chicken/Green Pea formula.  I did try other brands but the vomit appear to be worse.

Changing foods too quickly, especially dry food, can cause gastrointestinal upset.  You want to gradually introduce new dry food over a period of about a week or so.  Does the kitty throw up shortly after eating or is later in the day?  You might need to keep a food diary of what you fed and any reactions so you can rule out possible food allergies or a negative reaction to a certain ingredient.  For example, I had a cat who threw up every time we fed her any canned food containing fish.  It took me several months to figure this out, but I made the connection when I kept a diary on what we fed her.

A food diary is a great help. I use a desktop calendar and call it the "in-and-out" diary, since I jot down not only what food was given that day, but also stool frequency and consistency and any vomiting. It makes it much easier to pinpoint what may be triggering the problem.

Aw, poor kittens! And not fun cleaning up all of that vomit either. I can only suggest that you take your kittens to the vet. Vomiting, yes, can be a diet related allergen, but it can also mean other things as well such as parasites, food intolerance, eating too fast, hairballs, diet change, infection or eating indigestibles. Have then been treated for worms? A stool check? You may have to separate them for one night so you can figure out who is doing the vomiting. Vibes for you kittens!

How do you separate a cat when you only have 1 litter box? Last night, feed the same amount and took the dry food away.  Woke at 3:30 just to see if any vomiting had occurred..NONE.  Gave them their normal dosage of wet food again at 6am.  Will try this again tonight to see what happens.

I haven't had much luck with cats (one just died in August) due to heart issues and am afraid of losing another..

It's best to have at leats one litterbox per cat (ideal number is one per cat plus one extra), so maybe now would be a good time to pick up another box. Can't hurt.