The most common back-up o-rings are contoured on one-side only but I've seen them contoured on both sides. The contour is for the o-ring to sit in. On your filter tower top cap you'd have the contour pointing down and the o-ring facing the pressure.
Back-Up Rings
Wait a sec. You mean they both ride in the groove? The backup ring beneath the oring? Like with a 2nd stage piston ring and an oring underneath it?