Tim's answer referencing Marty Goodman offers a good explanation, but not really any options for users today who might need to make this upgrade.
The 3024 model can be modified with a simple PAL upgrade, available from Cloud9
If you have access to a programmer and the proper GAL, there's information on Coco3.com for another option:
If you get a Gray or White Large MPI (both have catalog #26-3024), then the procedure is different – you simply replace the 14L4 PAL with a properly programmed 16V8. You can’t use the equations presented above for a 26-3024. Instead, use these:
Name 26-3024 M.P.I. Upgrade ;
PartNo GCC-3024.01 ;
Date 6/1/2010 ;
Revision 01 ;
Designer J&R ;
Company GIMEchip.com ;
Assembly 26-3024 M.P.I. Upgrade PAL for CoCo 3 ;
Location U6 ;
Device g16v8 ;
/* ***************** INPUT PINS *****************/
PIN 01 = !FFXX; /* LOW For Any Address Between $FF00-$FFFF */
PIN [2..9] = [A7..0]; /* CPU A7 – A0 */
PIN 11 = RW; /* READ = 1, WRITE = 0 */
PIN 12 = E; /* E-Clock */
PIN 13 = Q; /* Q-Clock */
PIN 18 = !CTS; /* Cartridge Select Signal. */
PIN 19 = !SLENB; /* Input To Disable Device Selection. */
/* ***************** OUTPUT PINS *****************/
PIN 14 = !DBEN; /* Enables The Multi-Pak Interface Data Buffer */
PIN 15 = !IOR; /* Read the Slot Select Latch. */
PIN 16 = !IOW; /* Write the Slot Select Latch */
/* ***************** LOGIC EQUATIONS & VARIABLE DEFINITIONS *****************/
FIELD ADDRESS = [A7..0]; /* */
LATCH = ADDRESS:[7F]; /* Slot Select Latch@$FF7F */
IOR = LATCH & FFXX & E & RW; /* Active on $FF7F and E=1 and RW=1 */
IOW = LATCH & FFXX & E & !RW & !Q; /* Active on $FF7F and E=1 and RW=0 and Q=0 */
DBEN = (ADDRESS:[40..7F] & FFXX) # SLENB # CTS;/* Active $FF40-$FF7F or SLENB or CTS */
/* *****************/
The 26-3124 model can be upgraded according to this image from coco3.com using a 74LS10 and some soldering skills:

Standard CoCo 3 upgrade for the 26-3124 Multi-Pak Interface. The
integrated circuit is a 74LS10 Tripple 3-Input NAND. The trace
connecting pin 19 of IC1 to pin 52 of IC6 is severed. This circuit is
assembled and connected as indicated in the schematic. This circuit
restricts the address range of the M.P.I. data buffer to $FF40-$FF7F.
The data buffer is aso enabled on SLENB* and CTS*. BA7 is buffered
address line A7.
Once you make this upgrade, your MPI will only be address compatible with the CoCo 3.