There are some c64 games that consume multiple disks (such as Maniac Mansion, Ultima series, Defender of the Crown, etc). The current firmware doesn't offer any way to swap disks on the fly, so here are some alternative methods for you.

Crackers over the years have converted these larger games to cartridge (.crt) format, and these play very well on the c64 mini (particularly 'EasyFlash' cartridges). So you can try search google for “c64 <gamename> cartridge” and you should be able to hunt down such cartridge versions. You may also find large collections of c64 cartridge games provided in one zip if you hunt around ;)

NOTE: This method is for the c64mini. The c64 full-sized firmware already has a gui-based menu option to swap your disks when in “classic mode”.

So for the c64 mini, if you have upgraded to firmware V1.1.4 or above (highly recommended!), then you can use this new disk-swapping workaround technique suggested by Frederik Rosch in the Facebook TheC64 mini group:

You can use Multidisks by giving both images the same name. With this, you have the same savestate for one game. You have to make folders then (ex. “Side A”) to seperate the files.
I already tried it and it works. But it's a hassle if you have many sides and you have to copy in every folder the same cjm file.*

You have the game folder. Then, there must be a new folder per side. The game must have the same name in every folder. If you have to change the disk, save the game. Then, select the needed side and load the savestate you did before.

* “Hassle” is no more since firmware V1.2.0 - it provides default settings for all files (and folders) within a folder, by using same “thec64-default.cjm”.
More about it at official Setting a Default Configuration for Multiple Programs
Manual solution for “hassle” with cjm files mentioned here could be by using very simple Multi-cjm batch script (in conjunction with other tools on same page). It will create multiple clones of same cjm file, but of different (dedicated game) name. Other tools helps to merge games (and cjm files) in one folder and separate them to sub-folders later (after using script).

As an example of how to restructure your disk images into sub-folders:

BEFORE:

maniac_mansion_disk1.d64
maniac_mansion_disk2.d64

AFTER:

+ maniac_mansion/
|
+--+ disk1/
|  |
|  +--: maniac_mansion.d64
|
+--+ disk2/
   |
   +--: maniac_mansion.d64

NOTE: This technique requires firmware V1.2.0 or higher.

If you still don't have any luck with the technique above and the game didn't load properly, it may be due to needing to turn on the 'accuratedisk' setting. Try to create a “THEC64-default.cjm” within the main game folder, containing:

X:pal,accuratedisk,driveicon
J:2*:JU,JD,JL,JR,JF,JF,SP,1,SP,2,3,4,JF

FINAL_STRUCTURE

+ maniac_mansion/
|
+--: THEC64-default.cjm
|
+--+ disk1/
|  |
|  +--: maniac_mansion.d64
|
+--+ disk2/
   |
   +--: maniac_mansion.d64

When you are prompted to swap disks, press the menu button

From the menu, select:

Save the game state with this button:

Exit the game with:

Use the USB File Loader to open the disk you want to swap to:

Load the previous game state with:

Now press whatever key/button is specified in the game to start loading from the next disk (usually space, enter or fire button)

Some users have had success in merging files from a multi-disk game into a single larger capacity disk, such as .d80, .d81 or .d82.

This technique may work for some games and fail for others, perhaps due to copy protection and unique fast-loader logic added into some games.

It may also be worthwhile googling for .d80/.d81/.d82 versions of the game you are interested in, to see if anyone has done the work for you already.

If you do want to roll your own, there's some info on this page:

• How to make multi-game disks

This method is the most cumbersome, so isn't recommended anymore. All the same, it is documented here if you are still interested.