Almost, but it’s a bit different. To be precise, it’s possible to build an instrument that will do exactly what you tried to make, although in basic and quoted parts there are not pairs, but currencies, stocks, metals… For example, your instrument would look something like eur+gbp/usd+jpy. You are not allowed to put usd in basic part – that’s the only restriction. Your instrument consist of 4 others, but you can use more.