The repository will use the T-Engine configuration to choose which T-Engine will perform a transform. A transformer definition contains a supported list of source and target Media Types. This is used for the most basic selection. This is further refined by checking that the definition also supports transform options (parameters) that have been supplied in a transform request or a Rendition Definition used in a rendition request. See Configuring a Custom Rendition.
Transformer 1 defines options: Op1, Op2 Transformer 2 defines options: Op1, Op2, Op3, Op4
Rendition provides values for options: Op2, Op3
If we assume both transformers support the required source and target Media Types, Transformer 2 will be selected because it knows about all the supplied options. The definition may also specify that some options are required or grouped.