Hi.
You can use 1 Diggit Displays.... And thats the best technic, too. Professional Cockpitbuilders work mostly with 1 Diggit Displays.
The Datasheet of MaxTubes talk about 2x4 cause they designed the PCB´s in that way.
Basicly your MAX got 16 Lines that are needed to run you Display. 7x Segment Lines + 1x Decimal Point + 8x Common Cathode Diggit Lines..
In the Tubes this are split in 24 Lines ( 2 Blocks) Each block got 12 Lines .... 8X Segment/DP and 4x Diggit .
So the Segment Lines for Left and Right 4x1 Block are just duplicated .
*****
For your system:
If you plan to set the RAW Max Chips ( like this ...
https://www.ebay.com/itm/IC-MAXIM-MAX7219CNG-Max7219-IC-DRIVER-LED-DISPLAY-8DGT-24-DIP-/400551851482 )
then i will recommend to use ONE Chip for each Display.
Thats more tidy as you will split 1 MaxChip for example to 2 Displays ( 1x3 1x4 )
Don´t care about the empty 5 possible Diggits if you use only 3 Diggits by one Max.... For 0,99$ thats realy not a problem.
If you want to use more Displays to ONE Max than think about..... ONE Config MUST get to ONE Max.
Yo you can not easy say for example a 5 Diggit Display like MCP ALT should show 3 Diggits on Max A and 2 Diggits on Max B. Here you need then 2 Configa... A lot of work to set it and maby problems if you have bugs.
Summary....
I would use this chips direct on PCB .... Use 1 chip for every Display .... Use 1 Diggit Displays. If you need a 5 Digit Display use 5x1 Diggit Displays and set it together.
On PCB design lines from Max Chip to Display .... 8 Segments and Number of Digits for Daalines ( a 5 Digit Display got for example 8+5=13 Lines )
The Digit Lines must go individual to each Digit Connector spot.... The 8 Segment Lines can be used only 1 Time and can be chained from Diggit to Diggit on PCB.
Finaly i would do the Power, CLK , CS lines ( and DIN DOUT ) on the PCB too.... So finaly you have a Connector for all Displays to MEGA.
If you like it extremly professional you finaly can build a Pin Socket where you simply connect the hole Mega form behind. Then the Hole Element (Including the Mega) is without wires. You only get a USB Connector and all other things are on PCB.