Did you mean a two-PART check or a two-PARTY? I ask because I would assume that it is OK to dissassemble and reassemble the pistol while by oneself...
If you're by yourself, check it twice, using an alias the second time.
In my department weapons are issued as needed, so a two person check would be a better way of saying it. He could’ve used a student to visually verify if the weapon was empty. The idea is that one person might get it wrong , but two are less likely to.
The two party check is for when you are with others
In classes I've gone to, and whenever I teach, we use the two-party check, where the owner of the gun verifies that it is unloaded, and a second party double-checks (by sight and by sticking his finger in the empty chamber) that it is unloaded.
The reason for it is, you need to be double-sure when an error might endanger others.
The reason for checking by sight AND feel is that occasionally people think they see what ought to be there rather than what IS there