Please switch to a normal grass hay - timothy, meadow, orchard etc. Grass hay needs to be the main part of their food, so it’s essential they eat lots of it. It should be 80% of their daily food intake.
They should not be eating alfalfa hay at their age and certainly not until a year old - the vet has given you incorrect advice. Alfalfa is not a grass hay and is too high in calcium to be in their diet.
Alfalfa is only recommended to be given to pregnant sows and baby piggies before they are weaned at 3 weeks of age. After that, they don’t need alfalfa at all.
Veg is one cup per pig per day, around 15% of the daily food intake. Herbs and leafy veg is good but don’t feed high calcium veggies such as spinach more than once a week.
Carrot should only be given one tiny piece once per week as it is too high in sugar.
Fruit is the same and actually need to be in the diet at all. They can have one small piece once per week.
Tomato is too acidic so don’t feed that often either.
Pellets are the least important at 5% of the daily food intake. They should have just one tablespoon per pig per day and never allowed constant access to pellets. Too many pellets contribute too much calcium to the diet, they also can prevent them eating enough hay and can contribute to poor gut and dental health.
She may be filling up on too many pellets and alfalfa hay and therefore not wanting anything else so it’s important you change from alfalfa and make sure pellets are kept strictly limited to encourage her to have a normal grass based diet and some veggies.
In terms of the chasing, make sure a normal grass hay is available in multiple places around the cage so both piggies can have free access to it. The same with veg, make sure it is in two places and ideally scatter fed so both piggies can forage for it (which is great enrichment) and the dominant piggy cannot food hog.
Their one tablespoon of pellets can also be scatter fed.
After this, all you can do is offer veg and she will learn what is safe to eat over time. It may be that she had not been exposed to much before now and simply doesn’t know what to eat. Pet shops simply don’t tend to feed the variety that we do as owners so the piggies never learn before being brought home.
The dominant will have her pick of the best food and some chasing and dominance behaviours is entirely normal but if chasing becomes excessive and if the more skittish piggy loses weight through constantly being chased away from food not being allowed to eat enough hay, then it’s important you ensure there isn’t a problem with their bond.
Please do ensure you weigh your piggies every week as part of routine care as you cannot tell they are eating enough grass hay to gain weight simply by watching them.
The green links I’ve added in below will help you further
Long Term Balanced General And Special Needs Guinea Pig Diets
Sows: Behaviour and female health problems (including ovarian cysts)
Owners' Comprehensive Helpful and Supportive Information Collection
Its good you’ve got a big cage - two sows need 150x60cm.
Make sure you have multiple hides and that all hides have two exits so no piggy can get trapped inside one.
Also, have you checked they are both indeed female?
https://www.theguineapigforum.co.uk/threads/illustrated-sexing-guide-for-all-ages.156297/
