Can you learn Spanish and another language at the same time?

Yes, you can, but each language will usually progress more slowly than if you focused on one. It works best when the languages are quite different from each other, you keep separate routines, and you have a decent level in at least one. Two closely related languages started together is the hardest combination.

The honest trade-off

Time and attention are limited. If you have an hour a day, splitting it in half means each language gets half the hours. Since progress is mostly about hours, you'll reach any given level in each language later than you would alone.

Plenty of people do it anyway, because they want both, or they live in a multilingual setting. The trick is being realistic about the timeline.

The biggest risk: mixing them up

Related languages cause the most interference. Spanish with Portuguese, Italian or French is the classic case. Words that look similar but differ slightly start to blur together, and you find yourself saying Italian words in a Spanish sentence. If you're curious about this overlap, see Spanish vs Portuguese.

Languages that look nothing alike, like Spanish and Japanese, cause much less confusion. They also compete less for the same mental space, since the vocabulary and sounds don't overlap.

When it works well

Learning two languages is more realistic when:

One is already at an intermediate level. Maintaining a language you know takes much less effort than starting one.
They're quite different. Less interference.
You have clear separate routines. Spanish in the morning, the other language in the evening.
You have enough daily time. Roughly 45 to 60 minutes total gives each language a reasonable chance.

When to wait

I'd suggest holding off on the second language if:

You're a beginner with little time. A single language gets you to a useful level sooner, and that early win helps motivation.
You tend to drop things. Two habits are twice the chance to fall off.
The languages are close relatives and you have no strong base in either.

None of this is a rule. It's just what tends to make life easier.

Practical ways to do it

Separate them by time or place. Use one language in one setting so your brain links them to different contexts.
Use a third language as the bridge sparingly. Learning Spanish through a language you know well is fine, but it can add translation habits.
Keep your notes separate. Different notebooks or flashcard decks stop confusion.
Don't switch every day. Alternating blocks of a few days or a week can work better than constant swapping.

What about maintaining versus starting?

A common and sensible plan is to take Spanish to a comfortable level first, say lower intermediate, then add a second language while you maintain Spanish with lighter daily listening. You keep what you've built and still have energy for the new one.

For that lighter daily listening, Langtok (my app) gives you short Spanish stories you swipe through, with a streak, so keeping Spanish alive stays a ten-minute habit.

If you're unsure how long that first phase takes, how long Spanish takes to learn gives a sense.

My take

Doing two at once is fine if you really want both and accept a slower pace. But if you mainly want to speak Spanish soon, I'd focus on it for the first several months, get over the hump where it starts to feel natural, and add the other language after that.