I am an occupational therapist and a large part of my job is wheelchair/seating recommendations. It's actually not a good idea anyway to add an extra underpad on a wheelchair because every extra layer causes more pressure for your skin (which can lead to pain and skin breakdown depending on your level of mobility). Most good quality wheelchair cushions have the option of coming with an incontinence cover that is designed to protect the cushion without harming your skin. If you have one of those, you don't need another layer. If you don't, and you're interested in getting one, send me a message with a little more info about your mobility (like can you shift your own weight independently), sensation, and any history of skin breakdown and I could make some suggestions for cushions to try (I promise I don't work for a company that sells them).
The other thing to look at is whether your incontinence product fits properly and is changed often enough. If it is, that should actually be sufficient. Maybe talk to someone at a health supply store for advice on making sure you have the right product.

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