is it possible to be signed up to eoc and 07?

yea, but you can only win one at a time

do I only gotta gain xp in 1? because currently I only play 07 but im signed up for both.

yeah you have to gain atleast 1+ xp in either one,

if you gain 1+ experience on oldschool and 0 xp on rs3 thats fine (and vice versa)

but if you gain 0 xp in both eoc and oldschool for 2 consecutive weeks we will remove you from the competition

youre just entered in the competition, it doesnt matter which once you play