Does Brintellix in combination with Ritalin (methylphenidate) cause excitotoxicity because AFAIK they both increase glutamate levels in the brain.

Doses. need to be taken into account. At high doses, both medicines can cause behavioral excitement. This shouldn't be a clinical concern, assuming proper management, unless one has bipolar disorder. However, glutamate is not a primary target neurotransmitter. Generally, drug interaction concerns are often best managed by the prescriber, since individual differences in pharmacokinets are significant.
Caution. Question not clear, eg, ?AFAIK. Think you're asking about possible harm if combining meds. Ritalin (methylphenidate) added to Brintellix increases risk for serotonin syndrome and may increase blood level of Brintellix. Discuss with prescribing doctor. These meds require careful medical supervision.