Western pleasure is from the Western field. If by HUS you mean Hunter Under Saddle, I guess that's covered by both English and Saddle Seat... Though for me it's in the English field (have to double check with @JulianHuttner if the one in saddle seat is the same as the one I know from English) . Either way both your disciplines are from different fields.
If you decide you want to do the Athlete track twice, then no indeed your horse will not be able to gain points in futurity while he is still active in those careers. He of course can still be put up for stud, you just don't get points for it. Now, once you retire him from his athlete career(s), you could pick up the futurity career in one of your now empty slots. You would be starting from scratch if your horse is first ESRPG generation, but you could use the achievements you've already done off-career to get him points and meet requirements to level up fast(er). If that is what you end up doing, make sure not to cash in any points from scrapbook pictures you would like to use toward his futurity career as you can cash in points only once.
Now, indeed, once you retire a horse from a career, you do lose the progress you made in that career, so choose wisely. However if you make it to the end of the career, you do remain in the Hall of Fame for that career, even if/when you retire.
If you want to have the Futurity career right from the start, then you will have to choose one discipline as your discipline, and the other will be just for fun. You would still be able to enter the other discipline in shows up to the regional tier and get points toward the athlete career with it as well.