Hey minime1164!
basically double rush is using Sand Rush Drilbur and Sand Rush Sanshrew on a sand team so they can weaken eachother counters to eventually (hopefully) pull off a sweep. Common teambuilding cores which can help you get into the tier are:
Hipopotas + Drilbur + Lileep + Pawniard + Filler + Filler
imo this is the skeleton to a succesful sand team its not the only sand team structure by any means, but it is defintely a succesful and tested skeleton used by many lc players. Basically Hippo is needed for sand, Drilbur is the reason to use sand as it is just such a succesful sweeper and some even think its broken in sand (lol) Lileep solves some synergy issues, namely to water-types by having the storm drain ability. It is also extremely bulky in sand with the sp def boost from sand storm. Swords Dance Pawniard with Pursuit lets you have a good answer to Snover, sands biggest fear You can deal damage with pursuit to Snovers that switch out when they are locked into Blizzard, Giga Drain, Ice Shard, etc. Additionally, its a good subroost murkrow counter.
A common skeleton for balanced teams are:
Choicer Scarf Snover / Subroost Mukrow /Fighting-type / spinner / Murkrow counter / Filler
quite a few succesful teams use this Formula, iss's lc rising, and ebeasts balanced bucket variable does to iirc.
Scarf snover murders sand and has fantastic synergy with subroost krow since krow can easily scare out the Fighting-types that threaten snover and Snover can take out one of krows biggest counters, Chinchou. Due to the rock-type weakness a spinner and fightin-type are needed to remove rocks and threat rock and steel-types respectively. A Murkow counter is needed so murkrow doesn't tear the tean apart and the last slot is up to you! (well they all are really lol)