Custom Key Signature works only in Treble

How do I create a Custom Key Signature that works for both treble and bass?
I was able to create a Custom Key Signature (Ab & Eb; no idea what key this is). When I apply this to my music it works fine for the treble staff, but the bass staff signature comes out as Cb & Gb, because the flats remain IN THE SAME STAFF LINES. I saw in your user manual where custom key sigs do not transpose... maybe they should? Also, I tried to create a second custom key sig. for the bass but I can't apply two different key signatures.
Help?
BTW, I'm using Musescore 3.6.2.548020600, Rev. 3224f34 on MacOS Monterey 12.4 (Quad-Core Intel i5), if that helps.