The client and server are conditional partitions, which imply the different roles of the two hosts communicating.
In addition, a client-server may imply a very specific design pattern, which simply introduces fixed roles for two applications, but does not impose restrictions on the data transfer process itself.
Host-to-host - any connection with the same logical host roles.
Physically, it is still a connection between two hosts, using any technology - WCF or manual work with sockets.
On the other hand, under Host-to-Host, JP Morgan ACCESS® Host-to-Host SM may be meant - a specific trademark of a specific protocol with a specific encryption inside.