Well, everyone knows these two are best friends, but they seem to have a strange sort of friendship. Goten and Trunks are best friends, but rivals when it comes to sparring. Trunks and Goten have very different personalities, but somehow their characters fit together very well.

Trunks does tend to lead Goten around, but Goten doesn't seem to mind, but rather enjoy not having the weight of decision on his shoulders. Trunks, being the older and dominant of the two, usually decides on their actions... whether this is a good thing or not isn't always clear. Trunks does have a tendency to get the two of them into trouble. (For example... Mighty Mask, whose idea was that? ^_^) 

Trunks is a little spoilt and self asserted, and doesn't often listen to any protests Goten has to offer. He even makes fun of Goten, but I guess all friends do sometimes. Trunks' nagging Goten into following his example most probably helped Goten to become braver and more self assured.

Well, whatever innocence Trunks lacks, Goten certainly makes up for. They don't argue much, and whenever they're not sparring against each other, they work as a team in battle (...Or Goten just follows Trunks' instructions.). And of course, we can't forget how er... well  they fought as Gotenks. 

I don't think either of them could have worked well as a character on their own, Goten and Trunks just seem to go together.