顔の丸くて若い。。。?
Is it because the て form of adjectives is only used when describing the same thing?
In this case, "the same thing" is "女性".
Therefore, you can use "te form" to connect these two sentences, but the particle should be "が".
顔の丸い
女性 + 若い
女性 = 顔
が丸くて若い
女性 ( young woman who has a round face)
1. When two adjectives are combined
without "te form", you can use "の" as the particle instead of "が".
noun + の + adjective(basic form,)+ adjective + noun
examples;
胸の大きいきれいな女性 = a beautiful woman who has big boobs
足の長いハンサムな男性 = a cute guy who has long legs
背の小さい太った男性 = a fat man who is short
you can also say
胸が大きいきれいな女性 = a beautiful woman who has big boobs
足が長いハンサムな男性 = a cute guy who has long legs
背が小さい太った男性 = a fat man who is short
2. When two adjectives are combined
with "te form", you should use "が" as the particle after the first noun.
noun + が + adjective(te form) + adjective + noun
examples;
胸が大きくてきれいな女性 = a beautiful woman who has big boobs
足が長くてハンサムな男性 = a cute guy who has long legs
背が小さくて太った男性 = a fat man who is short