While we don't currently have a rule explicitly forbidding this, I feel it's sensible for me to step in as per section 6 of the manifesto. We do have a rule against alts impersonating other users, and the omission of primary accounts impersonating other users appears to merely be an oversight.
In the first instance, I will forcibly rename any impersonators I see [edit: and remove their avatars, if appropriate]. I would like to ask that people refrain from doing it. If we can't resolve this by a simple request, I'll be asking to start a policy discussion with other admins/mods to come up with a reasonable way of dealing with it.