They are old enough to have block and veggies. Start with blocks and introduce veggies only after they are eating blocks regularly. I'm curious why you won't feed them one at a time? It's normal for them to fight over food and that includes syringes during feeding.
https://thesquirrelboard.com/forums/...-Pet-Squirrels
Here's the squirrel food pyramid and instructions on offering veggies and what to avoid. Introduce one thing at a time so you will know if something causes diarrhea.
Make sure they have a hanging water bottle and/or also a shallow water bowl with rocks in it.