Studying multiple subjects at once usually goes wrong in a predictable way: the subject you find easiest or most interesting gets more time than it needs, and the one you're avoiding gets pushed to "later" until later runs out.
Audit where your time is actually going
Before planning next week, look honestly at how last week's study time was actually distributed, not how you assumed it was. Most people are surprised by how lopsided the real split is once they check it against a timetable rather than a memory.
Assign minimum weekly hours per subject
Rather than an open-ended "study for X subject sometime this week," give each subject a fixed minimum number of hours before you plan anything else. Treat those minimums as non-negotiable, the same way you'd treat a class you can't skip.
Rotate which subject gets the best time slot
If the same subject always lands in your sharpest morning hours and the same subject always lands in your tired 9pm slot, one of them is quietly getting an unfair advantage. Rotate which subject occupies your best-focus block across the week so no subject permanently gets your leftover energy.
Interleave instead of batching everything
It's tempting to dedicate whole days to a single subject, but research on interleaving suggests mixing subjects within the week — rather than one giant block each — improves long-term retention, even though it feels less efficient in the moment.
Leave one flexible slot for whichever subject needs it most
Fixed minimums handle the baseline, but some weeks one subject will genuinely need more attention — an upcoming test, a topic you're stuck on. Keep one unassigned block each week that you allocate based on need, decided at the start of the week rather than by whatever you feel like doing in the moment.