How to merge container disk2 with container disk1?

Since there is nothing on Container disk 2 you can delete it. Then see if you can grow Container disk 1. But it looks like BOOTCAMP may be in the way. In that case, you might need to do some gymnastics to backup the BOOTCAMP volume, remove it. Then re-add it after you cleanup your disk structure.

You should just create an APFS volume within the container as APFS volumes will share the free space within the same container. This means you don't need to worry about sizing or moving partitions around. For example, I was triple booting 3 different macOS versions using APFS volumes for each within the same container.

Check the Partition tab -- if the Bootcamp partition is between Container 1 & 2, you'd have to delete it before you can join the Container 2's space with 1.

As the 2 containers show different free space looks like when the second was created a fixed size wa choosen. Or maybe the problem is Bootcamp in between them that doesn't permit dynamic pace allocation as mr. Brickley said. If removing the 2nd container doesn't free up space into the 1st, then BC must be removed (and replaced, if needed).

But if I was in you, I would rebuild everything from scratch.

I would boot into Windows BOOTCAMP and backup your files somewhere. Then boot back into macOS and delete the BOOTCAMP partition. The make Container 1 larger leaving 100GB of free space for BOOTCAMP for when you reinstall BOOTCAMP / Windows. Then you can boot back into Windows and restore the data you backed up.