The sliding X fell out of favour as the shock load of the drop if one side failed would likely take out the other side. Limiting this shock load potential has been achieved by placing an overhand knot in each arm of the sling, so the lower carabiner could still slide a little but only drop a short distance if one side fails.
This allows a small change in the direction of the load, but stops any equalisation at the upper carabiners so reducing the dynamic nature of the sliding X. True equalisation is had when each of the three points has the ability to adjust to the forces involved, not only the direction of the pull but sharing the load equally between all points. Knotting the sliding X is a compromise and can weaken the sling, though not beyond the ability to take a fall as slings are rated much higher than these loads.
A simple solution is to ad two back-up slings to a sliding X. These allow the primary sling to move freely in a dynamic event (fall / load with changing directions of pull) and balance all these forces. Choose or adjust the backup slings so they are just a little longer than each arm of the sliding X, the resulting drop if one side fails is smaller than the knot method and maintains the dynamism of the sliding X.
It’s very easy with multipoint anchors with a fixed central point to get the load direction wrong so most of the force is on one piece of protection. The arrangement suggested here will give a truly equalising two point anchor using the slings you have at hand or carrying a few extra sizes; super light Dyneema slings are easy to bring along: small weight penalty for the benefit of being able to construct a truly equalising anchor.
Post script: very interesting discussion on the sliding X unfolded after posting this on the Chockstone forum, go to the gear lust thread and search Sliding X revisited.
Also just found this piece over at Rock and Ice: http://www.rockandice.com/lates-news/climb-safe-sling-strength-in-3-anchor-configurations, the sliding x configuration was beyond the capacity of the tensile tester in this small sample, where as the knotted samples broke.