Code says you must have break before make and no way of ever allowing both
to be connected at the same time. Those do not appear to provide that.
Basically, you need a large dptt switch, which is what the transfer
switches create for you.
Besides, with that sort of thing, you have no easy control of which ckts
are drawing power; you can't insure that some are off and some are on.
Unless you have a HUGE generator, that could end up being a pretty big
problem and hard on the generator to boot.
You'll need a transfer switch to meet fire and building codes, plus a
quick disconnect outside (here in NY at least) for emergency services to
use. This is also definitely an insurance issue; not something they'd
miss or overlook should a disaster occur.

I have a question. I have wondered about taking two of the breaker/fuse
boxes with the levers on the outside as opposed to having a breaker inside
the box. Mounting one right side up and the other upside down then
connecting the levers by welding a rod to each. That one when one is opened
the other would be closed. The problem that jumped to my mind was the fact
that there could be a small amount of time when they both COULD be connected
because the throw on one box might be a little longer than the other. But
this would be a fraction of a second and could be gotten around by making
sure your genny and the line breaker both are thrown before switching.