Implied consent can only be taken if you can prove that both parties could reasonably expect to be put together in a transporter accident. Unless this outcome was so common that both parties were knowledgeable as to the risk, you cannot imply that their consent is based on an ambiguous idea that transporters are dangerous. You might reasonably consent to a danger of being killed, but I don't see how anyone could have a blanket consent that involves two different species and a blossom from a planet in the Delta Quadrant that hadn't been observed before. In fact the only kind of consent documents I can see that Star Fleet might require is the consent for a Commanding Officer to decide your fate should you be incapacitated in the line of your Star Fleet Service, which is what we saw Janeway actually doing in the episode. Actually if you watch the episode, in the beginning, Tuvix was also actively helping the Doctor to discover a way to separate him back into his two components. Could that not be construed to be a lack of consent on Tuvok and Neelix's part. It is entirely possible that Tuvix was not emotionally stable in the first place, and that we actually saw the beginning of his breakdown before he was separated. But remember this is "continuing consent", and what we see at the end is that neither Neelix nor Tuvok asking to be put back together.