The characters are Hinata Miyakawa(at the cashier) and Hikage Miyakawa from Miyakawa-ke no Kufuku - also known in english as "The Hunger of the Miyakawa Family".
Hinata works in Gamers part-time to fund her doujinshi buying habits.
In addition to their cameos in episodes 12, 16, and 24 in Lucky Star as mentioned in a previous answer, There are corresponding cameos of Konata and Kagami in Miyakawa-ke no Kufuku, at episode 5 and 7.
There's never any signal from the shows that the girls know each other and it appears to be purely coincidental that they bump into each other on occasion. The do live in the same area which would account for this.
As well as the Konata & Kagami, the shopkeeper of Gamers and Akira Kogami also cameo in Miyakawa-ke.
Summary from ANN:
The manga is based on the Lucky Star Moe Drill game series and centers around big sister Hinata Miyakawa and little sister Hikage Miyakawa's impoverished daily life at home (due to Hinata's wasteful habits) and Hikage's life at elementary school.
The show started to air in April 29, 2013,
5 years after the first episode of Lucky Star was aired on April 8, 2007.
The original manga of Miyakawa-ke was first published in 2009, (while Lucky star continues publication slowly but surely) so it's unknown as to whether these characters were meant to have such significance, or whether they were only chosen afterwards for the spin-off.