No, most likely your existing 9 speed Shimano cassette uses a typical multi sprocket carrier ("spider") and so you cannot simply replace just the 34t with a 36t.

For tandem usage, try to get a cassette that has all cogs made from steel, not alloy. Steel will last a lot longer and less chance of bending one.

see:

https://www.bike-discount.de/en/buy/...0-11-36-625054