Clarification: the resulting level of the pet’s abilities will be the average of the levels of your fused pets.
So, in this instance, the level of the ability you specified (is that the first or second ability…? why’d you write it like that?) will be (70 + 35) / 2, which is rounded down to 52.
A pet’s “max level” is then determined by adding 20 to the first ability’s level. Let’s say that it’s 52: if so, your resulting “Legendary” pet will have a max level of 72.
So, uh, don’t fuse now, or you’ll lose out on a lot of possible levels. (the desired, highest cap for Legendary is 90)