Alice and Harrison are a gender-reversed, parallel-universe version of Garp and Helen. Just slap a mustache on them and they'd be evil twins.
Alice, like Garp, is a writer. In fact, Garp seems quite taken with her writing—he tells Helen that she's the "real thing" (8.95). She and Garp differ in one meaningful way, however: Alice can't finish anything. While Garp has a marathon-like stamina for writing, Alice can never get past the first few laps.
Harrison and Helen are pretty similar, too. While both work as college professors, the pair bond the most over the fact that they're both married to writers. This gives them a greater understanding of each other—for example, Harrison acts as Helen's "confidant" (14.152) when he visits her at Dog's Head Harbor. But they differ when it comes to things that really count, so while Harrison languishes in more affairs, Helen is able to rebuild her marriage with Garp.
We can look at Alice and Harrison as a "What if?" scenario for Helen and Garp. Despite their hardships, Garp and Helen never lose their love for each other. It's hard to say what keeps Alice and Harrison together, but we never get the sense that they tackle their many issues.