How to Share Interrupts on Different Com Ports Under DOS:
Most com boards will not share interrupts (our TC-400, TC-420, TC-800 & 9002E cards are an exception). If the board cannot share interrupts and you have more than one com port on the same interrupt (like COM1 and COM3) you can only use one port at a time. If you try to use two at once, the second one to be initialized will steal the IRQ from the first so that it will stop functioning. Assigning a different IRQ to the IRQ selection will get around this problem, but you must be sure and configure your communications software for the non-standard IRQ.
If you wish to share interrupts, and the card you have supports it, then the ports must be accessed through a com driver which can handle shared IRQ's, such as a fossil driver, or an OS/2 driver.
The following is a fossil driver you can download: