I don't say, one bodies the other One's spiritual truth; But I do say it's hard to lose either, When you have both.
In the poem's final quatrain, the speaker considers what he's up against, living a two-toad-life.
He's not saying that the internal-toad (the personal sense of obligation, duty, or honesty) creates the external-toad (the societal expectations or the actual time and energy taken up by work) or vice-versa. One toad doesn't give life, give body, to the other.
It's the co-existence that makes things so hard. It's dealing with these internal and external forces in conjunction that makes it so difficult to live a fulfilled life.
The moral of the story: work is a hard thing to get out of—so steer clear of toads.