Actually the 817/818 only allows for one optional filter. However it is an IF filter, meaning that once filtered out, nearby interference does not affect the gain of the receiver. Whereas audio filters are outside the AGC loop so a strong nearby signal can “pump” the AGC, confusing the audio you hear through the audio filter, but only in the case of really strong QRM. The ssb filter is a “nice to have” option, but the cw filter is a must if you are a regular cw user operating on a crowded band. If you are not a frequent cw user, you possibly don’t need the cw filter because you can still use the rig on cw with the ssb bandwidth. This is why those narrower filters are both optional, but as stated, there is only one socket for an optional filter.
But - there is a way of fitting both filters. W4RT offers (or offered) an option to allow both optional filters to be fitted to your radio. The mod required is a little more surgical and requires the stock ceramic filter to be lifted out of the board, a new board added to carry the two filters and so on. That option depends on the availability of the Collins mechanical filters which are no longer in production. Possibly obtainable on the second hand market.
For that reason, the practical option is probably that neat dual bandwidth internal switchable filter from sotabeams. or another the many audio filter options which are nearly all external filters, further complicating your portable station. At that point you may well ask, why didn’t I get a kx2?
Or… (this discussion can go forever) … Andrew VK1DA/VK2UH