For people who want to learn more than one foreign language at a time, choosing two unrelated languages, such as Japanese and Spanish, is a good idea because it reduces the confusion factor. However, it is still a challenging task, for example, because people typically have limited spare time after work or university-relate activites, because one of the languages is particularly difficult, or for other reasons.
The first thing to do is to work on your time management skills. As Donovan Nagel says, "If your time management skills suck then forget it".
Donovan Nagel also recommends focusing on one language at a time for a few days instead of switching between them duriing the day. (This is fine if you learn languages in your spare time, but a university student will need to work on their primary foreign language—in this case Japanese—at least five days per week.) The rationale between Nagel's advice is to allow short periods of immersion in each of the languages.
Nagel also recommends using a diary to allocate times of the day and week to each of the two languages. He adds, "Allocate times where you tell the rest of the world to piss off and leave you alone". You need to be ruthless with your time. Shannon Kennedy also mentions this: maximising study time, diligently sticking to a routine and a predetermined set of resources.
Another important aspect is motivation: you need to remember why you want to learn both languages. You should explicitly state the goals you want to achieve with these languages. (A purely emotional goal such as strong fascination is OK, as long as that fascination exists.) For people who are considering to learn two languages simultaneouly, Nagel adds, "Before you start, picture yourself 6 months, 1 year, or a few years down the track and answer honestly whether or not you’d still have the same passion and motivation to keep going with it."
Nagel also addresses the issue of fatigue:
When you go through periods of boredom or fatigue with one language (it happens to everybody), instead of not being productive at all you’ve got your other language/s to focus on for a while.
(Shannon Kennedy also mentions this.) For the current question, this is a bit difficult, because fatigue with Japanese would quickly lead to a serious backlog.
Shannon Kennedy also suggests waiting until you're stronger in one language before you add another. Bill Price gives the same advice; he found that for him "a solid A2/B1 is a perfect time to introduce a new language to the mix."
Some people suggest changing up where you study when you switch between languages. Shannon Kennedy does not do this; instead, she study wherever she is with whatever materials she has at hand.
Apply the Pareto principle–also known as the 80/20 rule– to your language learning activities: focus most on learning activities that bring the greatest benefits. This is definitely possible for the non-obligatory language (in this case, Spanish).
One thing to do if the second foreign language is not obligatory is focusing on materials that require less effort, e.g. listening to Spanish song or doing extensive reading with materials at your level (e.g. graded readers), instead of focusing on learning activities that require you to actively learn vocabulary and grammar. The advantage of this is that it reduces the risk of fatigue.