audio out, f/ both HDMI & Optical ???

I haven't gotten the cables yet but my cable provider told me that my set top box might not like giving me digital audio out from both the HDMI & Optical feeds at the same time.

Basically I asked if I used the HDMI cable to the TV can I then feed my AV amp with the optical for the surround sound and was told that the box doesn't like splitting the digital sound signal and I could have some issues.

Has anyone used this setup or have any opinions on this?

Thanks