It is what it is - a pass through (or duplicate) of the motherboard PDS slot, so you can add more than one card. Each one has to have a different slot ID - gamba's "SE/30 Corner" has more details on what works with what:

http://home.earthlink.net/~gamba2/se30_pds_multiple.html

Also:

http://www.biwa.ne.jp/~shamada/fullmac/se30_minitowerEng.html