Lincoln Crown Court heard how Darius Vasiljevas indecently touched one of the 15-year-olds and then used his weight to pin down the second girl before touching her.
It happened at a multi-storey car park in Gainsborough when Vasiljevas joined in with a group of teenagers playing in the snow.
Giles Bedloe, defending, said: “There were a number of teenagers out there together and they invited the defendant to join in a snowball fight.”
“During the course of play fighting with one of the girls he fleetingly touched her.”
“Later on he was seen to be grappling with the second girl. He took advantage of her.”
Vasiljevas, 27, of Lewis Street, Gainsborough, previously denied the two charges of sexual assault on 20th December 2009.
But on Monday he changed his plea and admitted both charges just as his trial was due to start.
Judge Sean Morris told him: “Young girls are entitled to have fun without being taken advantage of. That is what you did.”
“What began as a good laugh in the snow turned into an upsetting incident.”
He was given a 12 month conditional discharge and put on the sex offenders’ register for a year.
Vasiljevas must also pay £400 compensation to each victim and £1,800 towards prosecution costs.