ティファのクラウドに対する好感度が高い場合、飛空艇に残ったふたりは、互いを求める気持ちを確かめ合う
ティファ = Tifa
クラウド = Cloud
に対する = toward
好感 = favoribility (affection value in this case)
度 = degree
高い = high
場合 = event...
飛空艇= airship
に残っ = left over (remain in this case, the "ni" says where they remained, airship in this case)
たふたり = two people
互い = each other
求める = to want/desire
気持ち = feeling
確かめ = confirmation
合う = This goes with above confirmation, meaning they confirmed with each other (a scene can confirm something for the viewers while characters involve do not know, this tells us it was confirmed between the characters)
Put it together with the participles and you get:
"In the event the affection value toward Cloud from Tifa is high, the two that remain on the airship will confirm their feelings of desire for each other to one another."