Can you take oxybutynin and solifenacin together?

No. It won't be harmful if you do so, but you'll lose all solifenacin specificity on drug receptors. You'll get all oxybutinine side effects, without getting a benefit from using solifenacin. You should use higher dose of oxybutinine, or solifenacin alone. If you don't get enough effect from solifenacin, you should increase the dose to 10mg, or change for another medication.