Rule of thumb for cooking multiple things in a microwave at the same time?
For different food types there is no single correct answer, as it is very dependent on the specific microwave energy absorption and reflectance characteristics of each item (food and container) in the microwave oven

It's possible to put one item in which has a high absorbency and one with a low absorbency, and to have one fully cooked and the other still cold

e.g. A potato placed in a ceramic bowl, and a sausage on a glass plate, would result in a hot sausage, and a cold potato (assuming the sausage and potato are of the same weight)

For similar food types a general multiplication is required, but not exactly by weight, as each microwave oven has an 'efficiency to food and weight' exponential curve. You will find the result of this out by experience with the oven. This rule applies somewhat to normal thermal convection ovens too

e.g. 250 g of potato takes 6 minutes in your microwave oven, then 500 g may take 9 minutes not 12 minutes, and 1 Kg may take only 12 minutes, until you reach the equilibrium point of no further improvement (about 1 Kg for my domestic microwave oven)