Aquathon day Before First Oly?

Personally, I would be fine with the swim but wouldn't want to do 4mi of hard running the day prior to an oly.

Sure you can do it, but if you're hammering for 4 miles that will effect your TRI time the next day. If you're just doing the tri for fun and then yes you'd be okay.

I would skip it. There is no benefit to do it the day before a race. There is also the possibility of injury.

Depends on why you are doing both races. If you want to race them pass. If you want to do the Aqua as an easy work out and then race....I don't see any problem with that. But Your Oly will be slower because of it. If you are doing both as workouts and part of a total fitness program I would do both.

I have an IM coming up in mid Sept. So I have done several back to back events that I used as hard workouts. Oly/century on back to back days with a Sprint Tri 4 days later. Worked out fine. 10 days later and with some rest I was stronger.

I have a Sprint and Half IM coming up again on back to back days in July followed by a dbl century 5 days later. I don't see any problem with that either. My only goal is finishing each event feeling strong. Then a few rest days and back at it with another Half 4 weeks out. Then 6 weeks to the IM. I am not looking for PRs just better fitness and more endurance this year.

A 2 mile run/400 swim/2 mile run should be an easy work out for a Oly distance Triathelete. As long as you don't just totally hammer yourself on the aqua I would think it would be fun. But I wouldn't do it the day before if I had planned on setting a full on race pace in my first Tri.

I assume that Tri is your priority (A race?). Because of that the best thing to do the day before is a day off or a short easy workout.
On the other hand, it's great to have an experience of OW swim in that same lake before your tri. I sure wish I did a lake swim before my first tri.
Is there an aquathlon in the same series a couple weeks earlier?
Can you do an Aquathlon as a relay? Team up with a person who is up for a 4 mi run, and just do a swim part of it. Or run very very easy in the aquathlon, which is hard to do (for me anyway, I am too competitive)