The current recommended approach is what you’re currently using: merge them before feeding that to tcksift2. That does indeed entail duplicating the data, so I can certainly appreciate the motivation. It wouldn’t be difficult to support such an interface in tcksift2, but it without complicate its interface (and we’d likely rapidly be expected to replicate this for other commands too). My preference here would be to (finally) implement piping for streamlines data (previously discussed here), which would immediately allow this kind of operation, and many others besides…