You can easily create the position yourself by selling 1 contract of brent and simultaneously buying 1 nymex. You can create the chart on IB by using their create synthetic security chart (it's the third tab on the chart function).
edit: i go with various spreadbetters in the UK who have this set up as a single instrument. Having said that, one of them just simply sells brent and buys nymex, it shows as separate contracts in the position screen.
edit 2: sells brent on ICE and buys wti on ICE, not nymex