I think Yumega is flax oil (and starflower?), so not the same oil as Salmon Oil. The oils have slightly different compositions (Omega 3 is higher in Salmon Oil and I think Yumega is higher in Omega 6? (maybe someone who uses it will know more than me?)). They will have slightly different effects on the body and coat. Both are good for skin health, but the Omega 6 will have more influence on the quality of the hair.
I think some people do use both oils at the same time (especially in their working or older dogs as the salmon oil is particularly good for joints). I think the same company manufactures SalmoPet and Yumega, so maybe give them a ring...I am sure they can give you more info. than I can