The Merchant of Venice
Salerio and Solanio to Antonio and Bassanio
Salerio and Solanio provide a foil to Antonio and Bassanio. These two pairs represent very different types of male friendship. It's clear why Salerio and Solanio are friends – their cynicism and wit make them a perfect pair for each other. But it also casts a gloss of frippery or trivialness over their relationship.
By contrast, we're never really sure why Bassanio and Antonio are friends, and whether they're on equal terms of friendship. (Bassanio may be unwittingly using Antonio for his money, for example.) What is definitely clear is that Antonio and Bassanio both express very deep feelings for each other. Antonio literally puts his life on the line for Bassanio, and at the trial Bassanio declares that Antonio is worth more to him than his life, his wife, and the world. Their friendship, more focused on loyalty and sacrifice than personal gain, seems like something out of ancient Rome. Still, this is only speculation, insofar as they both have other motives.
Nerissa and Graziano to Portia and Bassanio
Nerissa and Graziano are a shadow of Portia and Bassanio's relationship, in an almost literal sense. No sooner does Bassanio announce his intention to go find a sugar mama than Graziano says he will follow suit. Graziano announces that he can marry Nerissa, since Nerissa said she'd follow the suit of her lady Portia. Graziano and Nerissa are thus married because Bassanio and Portia are married. Graziano's engagement to Nerissa is abrupt, to say the least, and we wonder whether the couple's love is even genuine. (We wonder the same about Portia and Bassanio's love.)
The ring affair is further evidence of this shadowing: Nerissa tells us she gave a ring to Graziano, just as Portia gave one to Bassanio, and both guys give their rings away. See "Symbols" for more on this.