How to cite paper with several co-first authors in APA style?
Short Answer: You should follow the standard citation style. E.g., "Anderson et al".

As far as I'm aware, there is nothing in APA style that speaks to an exception for the situation where the authors of a paper contribute equally or that set out multiple "first authors".

More generally:


Ultimately, author order is a thing that exists independent of contribution. It is often related to size of contribution. In particular, being first author is often related to making the greatest contribution. But disciplines vary substantially in the meaning of author order. And even in fields where first-author usually indicates greatest contribution, authors in other positions sometimes make greater or equivalent contributions. For example, in some cases the first and second author will have contributed equally. It seems more sensible to say that both authors contributed equally than to say that they were both "first authors", because at the end of the day, a reference has to have a canonical order to the authors. So whether the order of authors is determined by contribution, agreement, a coin toss, or a game of croquet, there is an order to the authors, and someone is first.
It seems strange that it would be the citing person's responsibility to adopt a citation style that captures every nuance of relative contribution in a given cited paper. An appropriate place to mention relative contribution of authors is in the author note in a paper. More generally, there are a lot of variants on the "co-first-author" scenario. In some cases, authors will note equal contribution or that author order was determined by a coin toss. Ultimately, this could all get very fuzzy.
It would make citations and references even more challenging to compile. Relative contribution of authors is not usually encoded in abstract and indexing services. And it is not encoded into reference management software such as Endnote or BibTeX formats as far as I'm aware. So it would be an additional hassle to keep track of these things.
The primary goal of citation styles is to unambiguously direct readers to sources. Managing author credit effectively is a secondary goal.