A few thoughts:
(1) maybe use a system-level heat exchanger (2P-2P) as a starting point, so you have a model that you can simulate with. What this block won't allow you to do is to change the physical size of the component, but is this something you absolutely need to do? Some reference data will get you quick set up for the system-level block.
(2) Use two 3-zone Pipe (2P) blocks, similar to the one below. This block already models the wall thermal mass and conductivity. You can set the external enviromental heat transfer to a very large value so it provide very small thermal resistance. This way, heat will go from one pipe to the other, both two-phase. The Heat transfer coefficient model, when set to Colburn equation, can be used to adjust the effectiveness of heat transfer between the two-phase fluid and the wall. You may need this to get good match of reference data. Further more, it's possible that you need more than one section of the pipe on each end. You'll need to look at the results and tell whether that's necessary. If it's a counterflow type of heat exchanger, it's quite likely you need two or more pipe blocks in series on each side.
