You cannot do you want with all criteria set to 10. If you want a criteria to be weighted double that of the others, then give it twice as much Maximum score. For example, let's say I have three criteria and I want #3 to be weighted double that of #1 and #2. I can set the Maximum score for #1 and #2 to be 10. Then I set the Maximum score for #3 to be 20. For the assignment I set the Grade to Type > Point and the Maximum grade to 100. When you grade a student the three criteria scores will be scaled as a percentage of the 100 point maximum for the activity. Use the same priciple for any number of criteria and keep the assignment grade set to 100.
So, it is like Emma says, make them out to different amounts.