Players with multiple positions

For example a DMR/MR

Are they just as good in both positions? Or slightly better in the position that comes first? So a DMR/MR would be slightly better at DMR??

Depends , if the player DMR/MR has way more points in the Attack than defense sure it may be better if he played MR and so on . if he is a balanced player ex: ( 41 40 42 ) he could play the same in the 2 positions .

pretty much what elite91 said. if example they are a mr/dmr. if there is more balance of skill points in attack, they would most likely play better as a MR, vice versa. Also, think of it with what their special abilities if they have one. If a DMR/MR has an ability like dribbler, they are probably better used as an MR.

Then how bout DR/DC? How do we know what is the best position for that player?

the best way to find out is to play them in each position a few times and see how they do. :D Some will do well at both but I find most of my multi role players do have a favored spot they do their best in.