I never had IB do that on boxes like this but I suppose it is a risk. The idea was more to do it at other brokers in order to get the best of both worlds, IB's admittedly low margin rates without their horrendous "customer service". I used to do this at OptionsXpress which is now Schwab, can't imagine it wouldn't work at Schwab now.
Not to barge in on this thread, I have created a fresh one to specifically debate box spreads at Schwab.
https://www.elitetrader.com/et/thre...-borrow-from-a-box-spread-with-schwab.359127/