What do you mean on "different downstream"? You have different downstreams already on the cards, you can set them to different frequencies when you are in "enable" mode on the cmts by:
configure terminal
interface cable 3/0
cable downstream frequency 370000000
exit
interface cable 4/0
cable downstream frequency 378000000
exit
This way you have set the first downstream to 370 MHz and the second to 378 MHz.
What do you mean on "different downstream"? You have different downstreams already on the cards, you can set them to different frequencies when you are in "enable" mode on the cmts by:
configure terminal
interface cable 3/0
cable downstream frequency 370000000
exit
interface cable 4/0
cable downstream frequency 378000000
exit
This way you have set the first downstream to 370 MHz and the second to 378 MHz.
Of course! Are you using upconverter ??