im not an expert but let me try to help.
the feeding guideline stated at the back of the bag serves as a "guide", you may or may not need to completely follow it. You feed based on your dogs' current state (figure, age, health, activity), not what the bag states. If you can see that your dogs are not in their ideal body condition, then adjust accordingly til you can see that it is already "enough". Let us bear in mind that some dogs like to eat a lot, free-feeding will most likely make your dog over weight or worse, obese. Satisfy your dogs' needs, not their wants.
I, like most dog owners, only mix 2 different kibble brands when my dog is undergoing transition (dog food change), otherwise, I would pick one over the other. If you are trying to keep track of how much calories and nutrients your dogs are taking, then mixing 2 different dog food brands will just give you a hard time.
I don't have any cheaper recommendations for you because Vitality for me, is the best among non-premium dog foods available locally. However, I recommend Acana. It is very rich in meat content and low in grains, you can check the ingredients yourself to justify that. Vitality roughly costs you 1670 for a 15kg variant, based on
this, Acana is only 1960 for a 13.5kg variant, reference
here. Though you are spending 290 pesos more, Acana will give you more value since it provides your dog better nutrition overall, and your dog takes a lot of meat and less grains. With Acana's more meat content and far better nutritional content, you are likely to feed lesser amount of kibbles to your dogs, so a 13.5kg variant of Acana will last longer than your 15kg Vitality Variant. You can check everything about acana and their products
here.
hope this helps.