P = VI, and I = P/V
in your existing situation you have
I = 35/12 = approx 3A (for 2 lights = 6A)
In the new case
I = 100/ 12 = approx 8A (for 2 lights = 16A)
What is your relay rated to?, you will need to check. probably more than 16A, probably something like a 40A.
Also check can the ballasts you have handle such a large increase from 35 to 100W?