The time it takes to heat up two microwaves of different sizes at the same time will depend on several factors, including the wattage of each microwave, the initial temperature of the items being heated, and the amount of food in each microwave.
Assuming the two microwaves have the same wattage and you are heating the same type and amount of food in each microwave, the heating time should be approximately the same. The wattage of the microwave is a critical factor in determining how quickly it can heat up food. Higher wattage microwaves will generally heat food faster than lower wattage ones.
If the two microwaves have different wattages, the one with higher wattage will typically heat the food faster. However, if you're heating different types of food or different quantities, the heating times can vary significantly.
For example, if you're heating two cups of water in one microwave and four cups of water in the other, the latter will take longer to heat up even if both microwaves have the same wattage.
To accurately determine the heating time for specific items and microwaves, it's best to follow the manufacturer's guidelines and use the microwave's timer accordingly. Additionally, you may need to adjust the heating time based on your personal preferences and the consistency of the food being heated.