Two topics in a sentence

白い箱はカウチの上に緑のランプは机の上にあります。

This seems wrong.... like a verb is missing from the first topic. I got this from a Japanese tutorial site. I'm assuming that I'm missing some grammatical rules. Could someone break this down for me?

That's very nomal Japanese.
But adding kutouten "、" is more better.
白い箱はカウチの上に、緑のランプは机の上にあります。
The duplicate verb "あり/ある" is omitted.
白い箱はカウチの上にあり、緑のランプは机の上にあります。

白い箱はカウチの上に緑のランプは机の上にあります。

Grammatically 「ある」 here works as a predicative verb for both 「白い箱はカウチの上に」 and 「緑のランプは机の上に」.
However, a punctuation mark 、 ( 読点 [touten]) is missing between the two parts.
cf. Another punctuation mark 。 is called句点 [kuten] .
句読点 [kutouten] = 句点 [kuten] and 読点 [touten].

If a sentence has many topics, or if a part for a topic is long, it's better not to omit a predicative verb.

When a set of words which has one meaning is partly omitted, that can make the expression unkind and/or misleading.
In your example, a set of words 「__ の上に」 shows a location, so it's better not to omit 「の上に」 here, i.e. it's better to avoid an expression like 「白い箱はカウチ、緑のランプは机の上にあります。」.

I'm no teacher myself but maybe my teacher's explanation would help. This is how we were thought this in my university:

The two 'wa' particles are comparison/contrast particles. So in your sentence, the meaning would be like "the white box is on the couch, (while) the green lamp is on the table."

I hope this helps some... again, I'm just a student myself, but this is how I was taught.