My Pujari told me never to put the dust pan and broomstick together in house & he also told me to inform my maid about this.
Any reason for this?
The only reason i can think of is this.
The dusts that are still on the broomstick are considered as pure. Whereas the dusts that are collected by the broomstick in a separate container like the dustpan (as you said) are considered as impure.
So, the Pujariji must have said that in order to prevent the contamination of something that is pure with something that is not.
The dust, raised by the ends of a broomstick, and happening to defile the bathing or drinking water of a person kept in a vessel, or touching his bathing apparel, tends to destroy bis religious merit on the moment of such contact or defilement.
Likhita Smriti's last chapter.
He, who is covered with the broomstick dust and the water sprinkling from hairs, [both] originating from a temple, is [said to be] sprinkled with the Ganges water. (313)