I'm pretty sure you all want a backstory of how UF Frisk met US Frisk, and how this ship even came up in my head.. well.. here it is!
UF Frisk was running away from the UF bros and everyone in the underground. She had Flowey with her and together.. they kept going until they saw a portal. UF Frisk jumped in it, and it closed.. that's when she met US Chara. They became friends but.. she saw another girl that looked like her.. but.. more "evil".. which was US Frisk.
UF Frisk picked up the courage to talk to US Frisk, and that's when a spark between the two started coming. US Frisk was trying to deny that spark.. (tsundere) but UF Frisk didn't.. she knew that the spark she felt was real.. but.. didn't know how to tell US Frisk her feelings.