As long as you monitor each cake for doneness individually, there is no reason not to bake them together—assuming they both need the same temperature.
If the sizes are radically different (in terms of the thinnest dimension, which is usually the depth) you may need to adjust temperature to control crust formation during the time it takes the cakes through to the center.
If the cakes are close in depth, they should bake in approximately equal times.
You may wish to use cake strips or aluminum foil to protect the thing easily burned permiter of a hemispherical cake.
Note that 150 C (300 F) is an atypically low temperature for baking a cake.
See related: Additional Cooking Time When baking more than one loaf of quick bread
