Quote from achilles28:
Do ECN's offer synthetic crosses on the majors?
Essentially all of the real ones do, just do the synthetic
yourself, simultaneously taking the prices in the appropriate
amounts. This will leave you in exactly the same position as
if you had traded the natural. On independent ECN's (EBS,
CurrenEx, Hotspot, Lava, etc) your buying poser is affected
the same either way. In captive in-house ECN's (IDealPro,
MB/EFX, Dukascopy, FXLQ, etc.) margin treatment of open
positions may be dependent on the route you took to those
positions.
You can, on most ECNS, denominate your trade in terms
instead of base, e.g. short 1mio USD/GBP and long 1mio
USD/JPY, which makes figuring out how much to trade a
little easier.