Two kinds of sharing here:
PCI slots sharing an interrupt. This is not a problem, not even with bus mastering devices - as long as their respective driver software is written to follow the PCI specification. One might think PCI has been here for long enough (seven years), but there are still some poorly written drivers out there.
The other one is mechanical sharing - that means that two slots of different kinds (PCI and ISA, ISA and AMR, ...) share a slot bracket position, meaning you can only fit one card at a time. Like when you have an ATX board (thus allowing seven slot positions total) with 1 AGP, 5 PCI and 2 ISA, then you'll have one position where you can either fit your fifth PCI card or your 2nd ISA card.
Regards, Peter