When we have two dogs in the home and one or both are resource guarding, environmental management (also known as prevention) is of utmost importance.
We must remember that management can (and often does) fail, so if this lesson seems like "overkill" it's because I don't want you to experience failure.
Once we have assessed the risk the guarding dog presents, and we have assessed the bite history , determining that it is safe to proceed with attempting to modify the behaviour, we set up a safe environment for both dogs.
Some forms of management will be temporary and others will be permanent. Below are some ideas in order to keep both dogs safe:
RESOURCE / CONTEXT
Dog guards full food bowl from other dog
Feed dogs separately using baby gates, crates, one indoors/one outdoors, a closed/secure door between them until finished, etc...
Dog guards empty food bowl from other dog
Once dogs have finished their meals in separate areas (see above) remove the bowls and put them away, allowing for a few minutes of decompression before they are allowed out again. Prevent them from running and cleaning the other dog's bowl or dining area.
Dog guards high value bones/chews from other dog
Only give high-value bones and chews when dogs are separated by baby gates, crates, one indoors/one outdoors, a closed/secure door. Once dogs have finished their high-value bones/chews remove the items and put them away, allowing for a few minutes of decompression before they are allowed out again.
Dog guards toys from other dog
Do not leave toys down when both dogs are together. Use management tools (baby gates, crates, exercise pens, doors) to keep them separate when playing with toys.
Dog guards "found" objects on walks (sticks, wood chips, bones, garbage, dirty kleenex) from other dog
Muzzle-train the guardy dog and/or walk the dogs separately until you have worked through this successfully.
Dog guards "found" items in the home like toilet paper, dryer sheets, hair elastics, socks from other dog
This is one of the tougher ones that requires 100% management in two ways: 1: the whole family is on board and is tidier than Marie Kondo. This is often impossible to achieve. 2: keep both dogs separated and only allowed to interact when you have scoured the area to ensure no "found" items can be ....found.
Dog guards guardian's bed from other dog
Do not allow either dog on the guardian's bed, or at least do not allow the guardy-dog on the bed. They lose this privilege with the first sign of guarding. Using a baby gate to the bedroom, crating or penning at night is the alternative. You cannot train when you are asleep!
Dog guards adult human family member from other dog
This one is far more challenging and requires tight management like "crate and rotate" where one dog is managed (baby gate, crate, exercise pen, door) while the other gets "snuggle time" and then you safely rotate them out so that they both get turns.
Dog guards child human family member from other dog
This one is the most challenging as a child is involved and they can be seriously injured if a dog fight breaks out near them. This one requires 100% management involving zero access to the child for the guardy dog.
Do we WANT to have to do these things? No. Everyone wants a free and easy life with dog. Do these small changes make a huge difference during training and beyond? Yes. Is it worth it? Absolutely.