All investigators have 2 hands . If the 1-hand slot is occupied and the 2-hand slot is free, it should be possible to use the 2-hand slot for a 1-hand item.If a 2-hand item is played, of cause one of the 1-hand items must be discarded.

Hence Roland should be able to have his .45 in one hand and his magnifying glass in the other., but if he plays his shotgun one of the two other items must be discarded.

I know that this is not exactly what the rules says, but that is the way I play when I play solo.

You're free to play however you wish, but officially there's not a 1 hand slot and a 2 hand slot, but two single hand slots and a two hand item takes up both slots so you can't use any other hand slots while using something like the shotgun.

I believe you are wrong, since what you are saying is that a 2-hand slot take up both the 2-hand slot and the 1-hand slot. I can find no rules saying that this is correct. The Rules Reference say that you have 2 hand slots available, one 1-hand slot and one 2-hand slot

Another matter is that you cannot use a thing from both slots at the same time, you don´t have 3 hands, but you can carry for example a shotgun and a magnifying glass.

No, mwmcintyre is correct.

Look at the rule book on page 11.

You only have two hand slots. The symbols below only shows what slots an item may be using.

Nope, I am 100% certain on this. You have two hand slots total. There is not a one hand slot and a two hand slot. This is a very common restriction in games. The two hand slot icon simply means it takes up both slots.

Thematically an asset being in play means you are actively using it, so you can not actively use a shotgun and a magnifying glass at the same time. Although let's not pretend the mechanics of this game are that closely tied to reality. You can be in a locked empty room trying to escape and suddenly - an ally appears magically!