For accounts > 100K (or something like that) they offer "portfolio margin". The rules are complicated and I don't pretend to understand them, but do not necessarily give lower margin requirements on mixed long/short portfolios - it depends on instrument volatility I think.
yeah, they do look complicated, looks like you would only know what margin was required when you actual open the positions
maybe i could give my current forward test positions to IB and IB could do a worked example on this thread maybe ?
belflan