I have traded on velocity and citifx and they have the option of aggregating the prices for you based on your order size, FCIntl is also a very good broker but they have always favoured QIB’s and institutions. Never traded with them because I felt that I will be realtively too small in comparison with their other clients and can possibly get not treated as well.
One way of doing that (which you have already figured out, is to slice your order and spread across multiple brokers. At the end of the day every broker / entity / liquidity pool has a tag number or identifier attached to it and all the limits (as per the best of my knowledge) is attached to these specific ID’s. So LP’s treat them independent of eachother.
In this specific case, I believe you will only get diversity of LP ID’s (because all broker’s clients, technically would be a sub-id or same ID as that of the broker). I dont specifically see how it will help you in getting better fills. As far as I see their liquidity is very similar (if not exactly the same) and if you are hitting the markets on both of them independently, its going back to the same set of LP (at best their agregators will give you some internal routing diversification, but not significant). So technically, you are splitting your orders only to be recombined again (atleast partially) at the LP level. Hope that I’m making sense
Essentially, if you look at Advance Markets (not an offshore broker but a kangaroo broker) you can do size with them but their spreads are not the best ones in industry.
Coming back to your point, I think you will have to both slice your orders and add delays to your orders (say 50 MS or so) if you want true diversification, this will in a way fool the LP’s that quickly move spreads for few MS and come back when they see significant sizes. But, if you are doing a hit-and-run strategy, you will have to be slightly more creative than that.
hope that helps