"Question: are double-trusses a trait that can be promoted through selection >

Yes.

"Question: would the seeds saved from the double-trussed fruit be more likely to continue the trait than seeds from "normal-trussed" fruits of the same plant>

The orthodox genetic answer: No.

Another possibility:

Since the plant only showed one double truss then it is possible that it was only an environmental effect.

More complicated genetic answer:

Even if the effect was environmental it may be possible through genetic assimilation to successfully select for plants that show an increase in double trussing. Such selection would affect the probability that a double truss will be produced when the environmental factor occurs. By continuing such selection for a number of generations it would be possible to produce plants that genetically produce double trusses even without the environmental factor. Double trussing would be considered a 'threshold' character which only appears when a certain number of factors are present (both genes and environmental) and the selection increases the genetic portion which effectively compensates for any lack in the environmental portion of those factors.

Less orthodox inherited possibility:

Inherited characteristics can be changed by genetic changes (that is in the DNA). But modern genetics has found that they can also be caused by changes in gene expression and sometimes those changes can be inherited. These are not changes in the DNA - they are sometimes called epigenetic mutations. The gene DNA sequence is normal but the gene is switched on or off at different times (than normal) and that causes differences in appearance (like DNA mutations).

It is theoretically possible that the double truss was caused by an epigenetic change in the cells of that truss only. There is a low (perhaps extremely low but unknown) probability that such an epigenetic change could be inherited or passed on through the seeds in the fruit of that truss only.