I'm currently learning Spanish in school while learning French in school. I've noticed that the vocabulary and grammar of both languages is structurally similar. I've had teachers tell me that I should focus on one language at a time while others suggest that I should continue learning both languages at the same time. Will learning both simultaneously only cause confusion or help reinforce similar concepts? I'm a native English speaker if that's significant.
Learning French and Spanish simultaneously can be both helpful and challenging.
There are a large number of words that are similar in both languages. An example is that the word for "cow" is "vache" in French, and "vaca" in Spanish. That's the helpful part. "Learn one version, and you basically learn both."
The challenge consists of distinguishing word pairs that are different. An example is "table," which is "table" in French and "mesa" in Spanish. It's helpful to note that in this example, the English and French words are similar (except for pronunciation), and it's the Spanish word that's different.
In my experience, "learning both simultaneously [can] cause confusion or help reinforce similar concepts" but more of the second than the first, which is why I choose to do so. Others may have different experiences, and make different choices.
I studied my second year of French and first year of Spanish during the same calendar year. This was no problem, and was actually quite helpful in learning Spanish because I no longer had to consciously recall basic French vocabulary and the similarities made my first year of Spanish go much easier. But the following year, when I was learning more advanced sentence structure, in both, I found it somewhat confusing.
I would suggest such a staggered approach if the languages are similar to each other and/or to English; even though English isn't a Romance language, it seems once one knows one of them the others come fairly easily (except Italian irregular verbs, imo).
Currently, I am learning both languages at the same time, one with direction, one without direction. As a result, there are definitely pros and cons to both.
One main facet is pronunciation. I've found it far more difficult to pronounce Spanish words than my peers, as learning French meant learning the ins and outs of its pronunciation, most of which are starkly different between both languages.
Another facet is similarities in the nouns themselves. There are many similarities between both languages in terms of words, as Tom Au pointed out in his answer, but there are also many differences. Distinguishing the meanings and usage only comes with practice, so I suppose in this sense it could be simultaneously easier and difficult.
I haven't learned enough grammar in each language to conclude whether the languages' grammars are similar, but from what I've learned, much remains the same between them.
Honestly, most of the difficulty comes from burning out on one language or the other. If you're motivated—and can stay so—in both languages, then this is not an issue; but it's very difficult to stay motivated when faced with such a difficult task.