Sowel die sypaneel as die agterpaneel is reguit.

Sowel ... as means both ... and.

So:

• Sowel die sypaneel as die agterpaneel
  = both the side panel and the back panel

It is a fixed pairing, so you normally learn it together as one expression.

Because each noun has its own article:

• die sypaneel = the side panel
• die agterpaneel = the back panel

Afrikaans usually repeats the article in this kind of structure, just as English says both the side panel and the back panel, not both side panel and back panel.

In Afrikaans, the present-tense form of to be is is for all persons and numbers:

• ek is
• jy is
• hy/sy/dit is
• ons is
• julle is
• hulle is

So even though English says are straight, Afrikaans still uses is.

Because Afrikaans very often writes compound nouns as a single word.

So:

• sy
  • paneel → sypaneel
• agter
  • paneel → agterpaneel

This is very common in Afrikaans. English often writes similar ideas as two words, but Afrikaans prefers one compound word.

No. They just look the same.

In sypaneel, sy is related to side.
It is not the possessive pronoun his.

So sypaneel means side panel, not anything like his panel.

Reguit is an adjective meaning straight.

In this sentence it comes after is, so it is being used predicatively:

• Die paneel is reguit = The panel is straight

That is normal in Afrikaans, just like in English:

• The panel is straight

This is normal main-clause word order in Afrikaans.

The whole subject comes first:

• Sowel die sypaneel as die agterpaneel

Then the verb:

• is

Then the rest of the sentence:

• reguit

So the structure is basically:

• [subject] + [verb] + [complement]

Yes. That would also be natural and means almost the same thing:

• Sowel die sypaneel as die agterpaneel is reguit
• Beide die sypaneel en die agterpaneel is reguit

Both mean Both the side panel and the back panel are straight.

The first uses the pair sowel ... as.
The second uses beide ... en.

Die can be both singular and plural in Afrikaans.

So:

• die paneel = the panel
• die panele = the panels

In your sentence, each noun is singular:

• die sypaneel
• die agterpaneel

But the article is still die.

Here it is written as one word: sowel.

In the expression sowel ... as, you should learn sowel as the normal spelling. It functions as part of the fixed pattern meaning both ... and.

So the correct form here is:

• Sowel die sypaneel as die agterpaneel is reguit.