It's funny from my point of view I understood that luffy and ace were both trained by somebody else…
We know this:
-Luffy and Ace wound up training together.
-Shanks makes fun of Luffy's handi-cap all the time.
-Shanks seems to be really worried about Ace.
-Garp left Luffy to train with someone.
-Ace calls Luffy "Lil Bro" and travelled across the Grand-Line just to see if he was alright.
Based off of points 2 and 3, it sounds like Shanks is much closer to Ace than Luffy. A real father wouldn't shadow his son (like Shanks' and Luffy's relation) just to make some other boy look better–and we know Shanks isn't like that.
How or where exactly Luffy and Ace trained is quite ambiguous. All that is implied is that Garp gave Luffy one disturbing, Lion-like child-hood, which created fear and animosity. We also know Luffy was always training himself to try and beat Ace--again, Shanks wouldn't scold Luffy like that by pitting him against Ace. In addition to the fact Shanks always avoided or ignored Luffy when he asked him to accept him into the crew; it seems like Oda is suggesting Shanks never directly trained Luffy or Ace, he must have just been the paternly guardian for them and made sure they stayed out of trouble.
So you may indeed be right, ero_seinen. I'm sure we'll learn more when Luffy gets to the pirate's summit (Gets shot by Stephen).