In reply to Max Eckert:
You criucially haven't said WHY you want to rig a top rope or WHO will be top roping or HOW MANY times it will be climbed, or HOW OFTEN you will be doing it.
The 'best' way varies depending on the situation.
With competent, experienced climbers who are only going to climb the route once or a handful of times, just use a couple of quickdraws and (as others have said) stop worrying about the second draw being loose.
With novice climbers or where the route will be top roped numerous times, the option you linked to (knotted sling and four screwgates) is probably much better. If it's going to be a regular occurrence, perhaps better still if you have a pair of steel screwgates for your ropes krabs as they'll last longer.
However, things also depend to some extent on the type of bolts and exact anchor set-up.
Where you have a nice anchor set with two bolts properly linked with a chain and a ring, back to back screwgates will often fit on the ring and offer a secure and simple option.
With two separate expansion bolts with hangers and rings, a screwgate on each ring often works great. In most cases the screwgates will be in a nice orientation (perpendicular to the rockface rather than lying flat) allowing the rope to runs smoothly and provided that is the case exact equalisation is not an issue. However with (Portland style) 6mm U staples using just a single screwgate on each often results in drag, rubbing or twisting of the rope or krabs that can move around. In that case (or any situation you aren't completely happy with how the two krabs are sitting) equalisation with a sling resolves any issues.
A final option, if you're using the same route on multiple occasions, is obviously to upgrade the existing anchor (assuming it isn't already suitable for intensive use) so you can just top rope directly from it. For example I've got a selection of high strength, stainless maillons, krabs and rings that I could easily use for that purpose.