Is it impossible to have more than one theme in a stanza in a poem?
A stanza can have no central theme, one central theme, or several central themes. At issue is that a "theme" is not actually something that inheres in a poem itself, but rather what a reader sees as common topics or ideas running through the poem. When one does thematic analysis, one tries to find cohesive elements and focuses on those these which seem to follow a certain pattern. However, a different reader or critic might well be interested in seeing a different pattern
"My Heart Leaps Up" by William Wordsworth is not actually a poem in stanzaic form, as there are no divisions separating it into two or more stanzas. Instead, it is written as a nonstanzaic or continuous poem. Because the poem is very short, consisting of only nine lines, it is thematically simpler than longer poems, focusing on the concept of aging, and, as is typical of Wordsworth, contrasting innocent joyful youth with an uncertain future. However, other themes such as religion and nature do appear in the poem.