You can, but if they are close in age, you can expect some fights - especially if there aren't enough hens to go around. Generally, it's recommended to keep roosters about two years apart... That way the senior rooster pretty much rules the roost until he winds up on your plate.
If you have enough space and keep enough girlfriends for them, they should find their own peace.
I've raised both Silkies and RIRs and I would not personally recommend it. RIRs are sometimes aggressive (especially the roosters), and as your RIR rooster matures, at some point he is going to seriously batter your Silkie rooster, possibly even fatally injuring him. I eventually completely separated all my Silkies from the standard breed flock because they were taking such abuse. The same was also true of my Polish, so I ended up keeping the Silkies and Polish together in a large run and the two breeds got along fabulously.