Interlock is mandatory
Yes, you need some sort of transfer switch or interlock. This is absolutely mandatory. You must provide a locking means which assures it is impossible for the generator to connect to utility, backfeed the utility wires, and kill linemen working 48 hours straight to restore your service (who are exhausted and not on their A-game re: their own safety practices).
With most panels this can be accomplished with a "sliding plate" interlock for under $100.
How to connect two inlets
You can't just wire 2 inlets to the same panel. That would be paralleling and many things could go catastrophically wrong with that. If you didn't dead-short this generator, you might dead-short the next generator which happened to have one of its NEMA 14-50 sockets wired opposite. Or worse, you invite a future nitwit to try to attach 2 generators to the building, which guarantees a dead short once the generators drift out of phase.
That is just made out of cubic nope. Nope. Nope.
1 inlet per house, as a rule.
However, bear with me. There's a "long way around" to a way to do it.
You can install a "Critical Loads Subpanel" fed from a simple "Knife switch style" transfer switch. (definitely not those stupid 6-8-10 circuit transfer switch; those are useless.) Thrown one way, the subpanel is fed from the main panel. Thrown the other way, it is fed from a generator inlet.
If you use a 3-pole transfer switch, which also switches neutral, then this subpanel can have its own separate inlet.
And if you use 2 of those -- 2 3-pole transfer switches and 2 subpanels - then you can do exactly what you are trying to do. Feed 2 banks of loads from 2 inlets.
Because you are switching neutral, the generator must provide a neutral-ground bonded output.