Is sun protection factor additive? If I use an spf15 and an SPF 20, do I get a result of spf35?

No. You will get an intermediate effect proportional to the amount you use of each. For instance with a 1;1 mixture, the protection will be about 27.5. They are not additive.
SPF. Mixing two different sunscreens to achieve greater protection. Don’t do it. Mixing an spf 15 with an spf 20 does not equal an spf 35. It’s not only wasteful, but you’re likely to have just concocted an ineffective product!