SeanTom.
You're on the right track. Adding a double dose (30mL) of 0-5-6 would be the same as adding a normal dose (15mL) of 0-10-12. Your fertilizer numbers are simply ratios listed as w/v.
The same way that adding 10mL of 10-10-10 is the same as adding 20mL of 5-5-5.
You're not going to double the EC of your entire solution, but rather double the EC of that particular fertilizer contribution.