The singular is stapper and the plural is made by adding -s:
This is a very common plural pattern in Afrikaans.
Afrikaans often leaves out die when a word like beide already works as the determiner.
So:
This works much like English, where we say both walkers, not normally the both walkers.
Because moeg is being used after the verb is.
In Afrikaans, adjectives used after a linking verb usually stay in their basic form:
So in Beide stappers is moeg, moeg does not change.
If the adjective comes before the noun, it may change form instead:
The sentence you would want in Afrikaans is:
That is also correct, but it is slightly different in structure.
Compare:
The meaning is very similar, but the emphasis and structure differ a little.
A rough English-friendly guide is:
BAY-duh STAH-pers is moogh
A few notes:
Pronunciation varies somewhat by accent, but that approximation is a useful start.