Multi card configurations fall under the SLI branding with Nvidia or Crossfire with AMD. These solutions are simply designed to increase performance by allowing multiple cards to perform render tasks in parallel, allowing for increased performance in theory.
In terms of value, it very much depends on the current generation of cards and each performance tiers price point. For example, at one point it was of greater value to purchase two Geforce GTX 970's as opposed to a single Geforce GTX 980. This was due to the fact that the pair of 970's would only be 10% more expensive than a single 980, but outperform the single card significantly. This varies generation by generation however.
The problem with multi card configurations is that performance does not scale linearly. Depending on the optimisation of the software in question, one may see 75% scaling, whereby they achieve 175% of the performance one one card with two identical GPUs. However one may also see reduced performance in cases where optimisation for the software in question is so poor that it has a negative impact, which is rare, but has occurred.
As for TANE, multi card configurations are not supported as N3V have not implemented the feature.
Jack