How to increase the size of my disc partition without deleting current one

You have, say, a 500 GB drive.
You make two partitions, one is 200 GB, the other is 300 GB.
Now what you want is two partitions of 300 GB each.

Where do you think you can get that extra 100 GB?????????
Pull a rabbit out of a hat?????

You order a large pizza delivered for three people. Just right. Then another couple of people arrive. Come in! Nice to see you! Hold on while I expand this pizza. Hmmm... maybe if I just cut it differently it will get bigger.

EDIT:
To the idea of making two partitions of 300 GB each from a 500 GB drive: Not possible.
To the idea of putting Mac formatted space into NTFS formatted space: Not possible.
The two types of space you can add to NTFS space are NTFS space and unformatted space. You cannot hide one file system inside another.
Solution: Buy an external drive, backup the Windows side, delete that partition, add the unformatted space to the Mac partition, and use Boot Camp Assistant again.

EDIT again:
I wish these PC geeks would go away. You can use partition software to add free space to a Boot Camp partition. The reason you must use Boot Camp to create the partition is that a Mac cannot boot to a standard MBR partition. Boot Camp makes a hybrid partition that Easeus cannot understand. Give up and do it like I suggest using a backup.