The federal limit for a tandem axle group on the Interstate is 34,000 pounds. Axles spaced closer than the tandem range are treated differently, and axles spaced farther apart are weighed as separate axles or as a longer group under the bridge formula.
Sliding the tandem on a trailer changes the spacing to the kingpin and the weight carried by each group, which is how a truck is brought legal on the scale.
Regulation: 23 CFR 658.5.