Scientifically, they're not all that different, actually.
They both rely on the interpretation of certain mathematical solutions to established models; how many additional assumptions need to be made for those solutions to be valid varies with the specific manifestation of the concept in question (there's a number of them for both the multiverse and time travel).
Neither is experimentally confirmed or proven, and in fact most models in our current understanding of them probably COULDN'T be proven experimentally as they'd require other mechanisms that are similarly unproven (e.g. exotic matter with negative energy density and so on).
They're plausible insofar as the math, in most cases, does not necessarily preclude them. But that's not that uncommon for frameworks of sufficient complexity, and just because the math supports it doesn't necessarily mean it's real. One popular example of this is the fact that mathematically there is no special reason time would progress the way we experience it rather than the opposite way round (i.e. simply changing the mathematical sign) but it clearly does not seem to be the case; even if most of our models of reality would work perfectly fine, mathematically, if time was flowing "backwards" (from our perspective).
This is the biggest problem of something like String Theory - it makes mathematical sense, the models work fairly well (again, depending on which flavor of model as there's many) and run into no obvious contradictions, however we find it exceedingly difficult to determine if they're actually REAL. In part because some propositions are probably untestable (at least given current knowledge), while others are testable in principle but entirely impractical to verify (requiring e.g. a particle accelerator of a size on the order of the distance between the earth and the moon). Which is really why it should be called String Hypothesis, but that's neither here nor there.
TL;DR: scientific "plausibility" is often not a rigorous barrier, as many things can be plausibly made to work with our current understanding of reality; however, merely being plausible isn't enough to believe something, and evidence is required - which is where many otherwise plausible propositions fall short.