I was really lazy.

main.cpp

#include <stdio.h>
#include <iostream>
#include <string>
#include <sys/stat.h>
#include <sys/types.h>

#ifdef __WIN32
#include <conio.h>
#define fstat _fstat64
#define stat _stat64
#endif

void splitFile(FILE* in, FILE* p1, FILE* p2)
{
	unsigned long long int bytes = 0;
	int ch;
	
	struct stat buffer;
	fstat(fileno(in), &buffer);
	
	unsigned long long int size = buffer.st_size;
	printf("%llu", size);
	
	while (EOF != (ch=getc(in)))
	{
		bytes++;
		if (bytes <= size/2) {
			putc(ch, p1);
		} else {
			putc(ch, p2);
		}
		if (bytes % 100000 == 0)
			printf("\rwrote %llu bytes out of %llu", bytes, size);
	}
	printf("\rwrote %llu bytes out of %llu", bytes, size);
	fclose(in);
	fclose(p1);
	fclose(p2);
	printf("\nDone writing file.\n");
}
void combineFile(FILE* in, FILE* p1, FILE* p2)
{
	unsigned long bytes = 0;
	int ch;
	
	struct stat buffer;
	fstat(fileno(p1), &buffer);
	unsigned long long int sizep1 = buffer.st_size;
	fstat(fileno(p2), &buffer);
	unsigned long long int sizep2 = buffer.st_size;
	
	while (EOF != (ch=getc(p1))) {
		bytes++;
		putc(ch, in);
		if (bytes % 100000 == 0)
			printf("\rwrote %llu bytes out of %llu for part 1", bytes, sizep1);
	}
	printf("\n");
	while (EOF != (ch=getc(p2))) {
		bytes++;
		putc(ch, in);
		if (bytes % 100000 == 0)
			printf("\rwrote %llu bytes out of %llu for part 2", bytes, sizep2);
	}
	fclose(in);
	fclose(p1);
	fclose(p2);
	printf("\nDone writing file.\n");
	
}

int main(int argc, char** argv)
{
	FILE* in = fopen("rld-thesims4.iso", "ab+");
	FILE* p1 = fopen("part1.bin", "ab+");
	FILE* p2 = fopen("part2.bin", "ab+");
	
	int selection = 0;
	std::cout << "1=slice file\n2=combine file\nselection: ";
	std::cin >> selection;
	
	switch(selection)
	{
	case 1:
		splitFile(in, p1, p2);
		break;
	case 2:
		combineFile(in, p1, p2);
		break;
	default:
		break;
	}
	
	std::cout << "<ENTER TO EXIT>";
	getch();
	
	return 0;
}