Multiple Filters for Single Object in SVG

I know this is a very old question, but for people referencing this, you can also just separate your filter functions with spaces - filter="url(#f1) url(#f2)" should work

See: https://css-tricks.com/almanac/properties/f/filter/

You could add multiple effects in one filter, however if you want to stack the filters up, first group the object and then apply the other filter to your object.

<g filter="url(#f2)">
<rect width="90" height="90" stroke="green" stroke-width="3" fill="yellow" filter="url(#f1)"/>
</g>