Merge two 8 bit arrays to one 16 bit array
It's better to avoid dynamic memory allocation on an Arduino, with only 2 KB of SRAM memory.

Note that your arrays cost 2 (bytes/int) * (8 + 8 + 16) = 64 bytes. 
And it seems you are only storing booleans, which could be stored in one bit. So consider another alternative (I will not work that out, since maybe memory consumption is not a problem).

However, an easy solution is to use uint8_t.

Also, I would use a simple memcpy. Very naively it would look like:

uint8_t summand_one[8];
uint8_t summand_two[8];
uint8_t summand[16];

void setup()
{
}

void loop()  
{ 
  memcpy(summand, summand_one, 8);
  memcpy(summand + 8, summand_two, 8);
}


Using some constants for the sizes, you get more flexible/readable and maintainable code:

static const uint8_t SummandOneSize = 8;
static const uint8_t SummandTwoSize = 8;

uint8_t summand_one   [SummandOneSize];
uint8_t summand_two   [SummandTwoSize];
uint8_t summand_merged[SummandOneSize + SummandTwoSize];

void setup()
{
}

void loop()  
{ 
  memcpy(summand_merged                 , summand_one, SummandOneSize);
  memcpy(summand_merged + SummandOneSize, summand_two, SummandTwoSize);
}