In addition, 2-legged SMART routed guaranteed US stock/option and US option/option orders can be executed by IB on different exchanges, where each leg is routed separately. Any risk of resulting execution that does not satisfy the required order ratio is taken over by IB. _ They are saying they do it. I guess if their system sees enough liquidity, on multiple exchanges, they will route to different exchanges. I assume that order will also get done in the complex order book, or with Timberhill, the trading arm of IB. I could be wrong, but the only time I see the order being split up, is if Timberhill likes one leg of the spread, but not the other, they will sell part, and route the balance. I would rather have control of the order so I know what's happening. Why don't you ask them what % of there orders are done this way. I don't think they will tell you.