If the machine has an ISA bus, you will not be able to share interrupts, in the
vast majority of circumstances (see below).  That it works for DOS is only a
fluke of the hardware, and is not an indication that it will work under any
other operating system.  In fact, if it works at all under DOS, you're (a) lucky
and (b) likely to be surprised when, someday, it doesn't anymore.

EISA and MCA both rectify this problem (the latter being of no use to *BSD
users, of course), and a few particular specially-designed ISA boards are
available that will share interrupts, such as the AST 4-port card.  I've seen
documentation for standard serial ports that implies that interrupts can be
shared, but that documentation was just plain lying.

--scott