One example that says it is "OK" and preferable if done right:
LFD Hybrid Speaker Cable
From Dr. Bews the maker of the fine LFD amplifiers comes this novel and superior speaker wire.
To quote from the website:
" ... LFD Grainfree biwire is a biwire cable only, since it uses different conductors for bass and tweeter, i.e. multi-strand Grainfree copper for bass and solid core Grainfree conductor for tweeter. This is the most popular LFD loudspeaker cable since it has a full bass with a warm mid and controlled top.
This cable is very good and the Signature version uses a Grainfree silver conductor instead of copper for the tweeter conductor.
The Signature version is reference standard regarding sound quality despite being relatively cheap. The Grainfree biwire cables are VERY good choice for solid-state equipment ..."
Extracted fom the Gene Rubin website:
" ... This cable uses PC-OCC multi strand and LC-OFC ribbon that is relatively expensive and labor cost is high (its a pain to make), but its the only way the makers have found to make a really good sounding single wired speaker cable.
LFD have been trying to manufacture this super single wired speaker cable from copper for nearly 10 years. The LC-OFC ribbon gives the very good bass and the PC-OCC multi strand gives a nice mid-band/top, either cable made as a single wired speaker cable have issues at the top end or the bass. It might be a mad idea, but it works!..."