I've seen it done too - 15 and 2x7 ltr- not sure what the diver was trying to achieve but far to heavy and I would suggest an accident waiting to happen.
There are better ways of rigging kit and diving to the sorts of depths where you need to employ such strategies - as wisely suggested above if your doing the sort of dives where you intend to use decompression procedures then really you should really be on twins.
There seem to be some very good deals around at the moment on 2nd hand fabers
Surely your instructor on your course can give you some guidance upon better configurations?