Is it possible? I don't think it is impossible, but I would imagine that it is quite unlikely. Merges can only be done by CMs, and before anything happens the user has to "prove" that they control both accounts. I'm not certain of the details, but according to the Help Center:
... After you contact us, the Stack Exchange Team will reach out to verify that you own both accounts. If we can confirm your ownership, we will initiate a merge.
Likely the "reaching out" will be done via sending emails. And one can certainly imagine that two users could coordinate between themselves to provide all of the correct responses.
But one's usage of math.se gives data to SE which is stored. If the CMs feel that something fishy is going on, they very well could (and should, IMHO) investigate further. They might then find, for example, that one account primarily uses an IP out of Pisa, and the other account an IP out of Tel Aviv, and often the two accounts are visiting math.se from their respective IPs at the same time. I imagine that this sort of thing would give the CMs pause before initiating the merge.
Note, too, that sharing a single math.se account among multiple people would appear to be a violation of Terms of Service:
Subscriber certifies to Stack Exchange that Subscriber is an individual....
Doing so could very well result in a suspension of the account, or possibly its deletion.