Good day, I am focusing on learning Arabic ... but I see that there are many similarities with Hebrew. Would this be a good pair to learn at the same time? If so, why? If not, why not? Thanks.
I doubt you'll find any definitive answer to this.
Purely anectodally, I find that my knowledge of a language seems to degrade faster at beginner levels than at more advanced levels. For example, my Japanese is no higher than A2. I had to postpone my studies for about a year, during which time I had virtually no exposure to the language at all. When I finally returned to studying the language, it felt like I had gone back perhaps not to 0, but close enough. In contrast, I can spend years not using languages I have already studied up to C2, and when I come back to them I feel, albeit perhaps a bit rusty, still entirely functional.
I liken it to pushing a boulder up a hill. At the bottom of the hill the incline is pretty steep. Further up, the incline is gentler, and you start to see some platforms peppered along the way. If you drop the boulder while the incline is still steep, it rolls all the way back down to the bottom. If you drop it higher up, where the incline is gentler, it'll roll back down slower and eventually stop on one of the platforms.
Whether you can afford to push several boulders up that initial steep incline at the same time depends on a variety of factors. But if life throws you a curve ball, you're more likely to have to drop one of those boulders and let it roll all the way back down than if you just had one boulder to care about.
Of course, this is just based on my own experience, and it's nothing more than a metaphor I happen to find useful in my own language learning journey. To me, the first platform (after which I feel comfortable taking on a new language) seems to be roughly around a strong B1 or a weak B2. Since I'm not comfortable taking the risk of pushing several boulders up the hill at the same time, I only ever had one language below B1/B2 at a time.
For the opposing view, you may enjoy reading Frederick Bodmer's The Loom of Language. He argues in favor of learning closely related languages (which would be the case of Arabic and Hebrew) at the same time, with an approach based on recognizing the patterns of diachronic linguistic evolution. For instance, taking Latin, Italian, Spanish and Portuguese, you may discover certain patterns in how the phonology and spelling changed. Say, filio-figlio-hijo-filho or folia-foglia-hoja-folha (in the same order as I listed the languages above). From that you can derive some patterns, how the Latin "f" became "h" in Spanish, "li" became "j", etc. The book includes more detailed information on how Bodmer thinks you should proceed (what parts of language you should learn first, etc.). The benefit is that you are made aware of certain connections between languages you may not have picked up on otherwise. The downside is that there's a risk you might overgeneralize these patterns and apply them where they're not applicable.