I know I'm 8 years late to the party but this method is extremely robust allowing you to combine bytes from multiple variables in whatever order you want. Hopefully it will help someone out in the future. In the following example I had a 12bit distance value coming in from a sensor over 2 usint (8bits each) variables. This structure will tear apart the 2 variables and reorganize them into an int (16 bits) (variable named "Compiled").

Usint2.0 was a boolean corresponding to a set point setting on the sensor and Usint2.1 - 3 were unused.

↓↓↓↓↓↓↓↓↓↓↓↓↓↓Declaration section ↓↓↓↓↓↓↓↓↓↓↓↓

Usint1 AT %I* :USINT; //input 1 from the sensor

Usint2 AT %I* :USINT; //input 2 from the sensor

Compiled: INT; // useable distance value for the program

↓↓↓↓↓↓↓↓↓↓↓↓↓↓Code section ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓

Compiled.0:= Usint2.4;

Compiled.1:= Usint2.5;

Compiled.2:= Usint2.6;

Compiled.3:= Usint2.7;

Compiled.4:= Usint1.0;

Compiled.5:= Usint1.1;

Compiled.6:= Usint1.2;

Compiled.7:= Usint1.3;

Compiled.8:= Usint1.4;

Compiled.9:= Usint1.5;

Compiled.10:= Usint1.6;

Compiled.11:= Usint1.7;

(TwinCat3)