I don't think I did a good job of explaining the layout.
There are three hatches in the main salon floor. A big one in the center and two much narrower hatches on each side of it. If you open the single large hatch in the center with twins you step down in between the engines, there is lots of space there. If you open the center hatch with a single, the engine is directly below you, and I don't think (I could be wrong) you can slide in around it without lifting the side hatches. The port side hatch is under the couch (or chairs in some boats).
I seldom open the side hatches, I just go through the main hatch and climb around either forward or aft of the engines. There is room to sit in front of either engine, and decent space behind them, though the shafts/trannys are in the way.
As far as the speed thing goes, as I said in my first post I don't want to rehash the old single vs. twins argument in general. But a 20 to 30 knot boat is an entirely different animal than a 15 knot boat. They come with different issues costs etc.
Coming from our old Gulfstar, we wanted a boat that could comfortably run at least 15 knots when needed. I wasn't remotely interested in sea rays or other Euro styled type boats. We looked at a few sportfish, but didn't like the layouts. I like to fish, but that is not the primary purpose of this boat.
The Mainship 400 with twins checked all of our boxes. Quite simply, the single did not.
Other than slightly higher costs I see no disadvantage in having a boat that is capable of running economically at 7 to 8 knots when desired but can also run close to twice that speed when needed.
Again, I'm not speaking of every boat. But in the Mainship 400, the twin is a better boat, there really isn't a logical argument otherwise.