I think this is a good start to what should be a strong series. Already looking forward to the next one.
As for flaming and frost not being active at the same time, RAW states:
Upon command, a flaming weapon is sheathed in fire that deals an extra 1d6 points of fire damage on a successful hit. The fire does not harm the wielder. The effect remains until another command is given.
And:
Upon command, a frost weapon is sheathed in icy cold that deals an extra 1d6 points of cold damage on a successful hit. The cold does not harm the wielder. The effect remains until another command is given.
It is a command to make it a flaming weapon. It is flaming until another command is given. It is a command to make it a frost weapon. In other words, as another command, commanding it to be a frost weapon ends the flaming effect and vice versa.
As for flaming and frost not being active at the same time, RAW states:
Upon command, a flaming weapon is sheathed in fire that deals an extra 1d6 points of fire damage on a successful hit. The fire does not harm the wielder. The effect remains until another command is given.
And:
Upon command, a frost weapon is sheathed in icy cold that deals an extra 1d6 points of cold damage on a successful hit. The cold does not harm the wielder. The effect remains until another command is given.
It is a command to make it a flaming weapon. It is flaming until another command is given. It is a command to make it a frost weapon. In other words, as another command, commanding it to be a frost weapon ends the flaming effect and vice versa.