Execute two chunks of code in sequence but the code itself in parallel
Probably the simplest method is to remove the data dependency, so you would have

slv_reg2 &lt;= slv_reg0 + slv_reg1;
slv_reg3 &lt;= slv_reg0 - (slv_reg0 + slv_reg1);


in the same process.  This will run in parallel, and the adder should be reused.  Well, actually it won't in this case as the second expression should get simplified to just -slv_reg1, but with different equations you might not be able to make that simplification.  

Now, if chaining these two operations in the same clock cycle is too slow, then what you can do is add a pipeline register, like so:

slv_reg0_delay &lt;= slv_reg0;
slv_reg2 &lt;= slv_reg0 + slv_reg1;
svl_reg2_delay &lt;= slv_reg2;
slv_reg3 &lt;= slv_reg0_delay - slv_reg2;


In this case, you'll get the result in slv_reg2_delay and slv_reg3 with a 2 cycle pipeline delay.