Honestly I don't know how to code something like that, but for a cheap easy effect of that I would probably just use ent_morph. Keep my existing player going, but make him look like the other character. Then of course switch the other character into the character you were just playing as. Unless of course, the way they play and control are completely different. I do doubt that from what you're saying though since you mention the templates.

However this is only if you're going for a second character to play as or something like that. If they are just following a path or doing something of interest that you want the player to see. There is a cam path wdl in the aum magazine that can help you out. You can even set it to look at the other entity at all times. So that should work perfectly.