Resource guarding and food/toy aggression toward other dogs is completely separate from resource guarding and food/toy aggression toward humans. Often times, a dog will tolerate a human sharing access to a resource much more than they will tolerate from another dog. So your test of reaching in or taking their food bowl away doesn't really indicate anything with respect to dog-dog resource guarding.
For cases of inter-pack resource guarding, I recommend careful and structured management of the environment and all things in it that the dogs consider resources. All valuable food items should not be left out. Dogs should be fed in seperate areas and supervised until they are finished. All food bowls should be put down for meals and then immediately taken up. If the guarding is bad, sometimes dogs will guard the area they are fed in or food is prepared in even when food isn't present, so in those cases, feeding in various areas and keeping the dogs out of the prep area is recommended.
If they guard toys, you either need to keep toys away or provide so many toys that there is always plenty to go around. Buy and rotate toys in multiples so that there is never just one of a particular "new" toy.
If they guard access to the area around a person eating you need to keep them in a different room from people eating. Baby gates work wonders. Additionally, you need to start work on teaching a rock solid "down/stay" so you can keep them at a distance if they are in the same room as someone eating.
Typically, these dogs are the same ones who don't wait their turn to get treats when the other dog is around and will be grabby and even squabble among each other or all out fight if treats are doled out next to each other. For these dogs, a rock solid "sit or down/stay" needs to be gotten individually and they need to learn to wait for treats to be offered and only allowed to "take it" on verbal command. Next step is having both dogs in "sit/stays" across the room from each other while you walk from one to the other offering treats for them to "take it" each waiting their turn. Once they are extremely good at this exercise, you start having them gradually sit closer and closer to each other still having them wait their turn and only getting the treat when offered to "take it". (This can also be done with two people, each in charge of a dog.) So basically, if one dog waits and watches the other dog while it is getting treated, the reward is getting their own treat.
Additionally, really start watching your dogs for early signs of a fight about to start. Usually the dog who has the item will freeze, give a sideways "hairy eyeball" to the approaching dog, lower its head over the item, cover it with its paw or other part of its body, position its head or body between the item and the approaching dog, try and consume the item faster, move to a different location with the item, give a silent lip lift, give a low growl, air snap in the direction of the approaching dog, or any combination of those behaviors before an actual fight takes place.
