There are two ways you can do this:
1) Using shifting and masking.
2) Using the BitConverter:
http://msdn.microsoft.com/en-us/library/bb384066.aspx
In both cases, your numbers are in the wrong order so you'd have to prepare them. The first two bytes don't make 1, they actually make 256 in the order they appear. You'd need to reverse that order to get the correct number.
If you're doing this with shifting, the result happens naturally. You can loop through the numbers (in steps of 2). Then you can OR (|) the number into a destination, shift it left by 8 (the size of a byte), then OR in the second number.
If you're doing it with a bit converter, you'll need to copy the first byte into the second element of a new byte array and the second byte into the first element of that array, then run it through the bit converter.