What happens to drug metabolism when CYP450 enzymes are presented with two substrates
I think the OP is asking the following:


  If an enzyme is not specific one substrate, but is simultaneously presented with a pair of substrates, which one 'wins' and how do we decide?  


For example, if an enzyme is presented with a racemate such that the D-form has a 10-fold lower $K_m$ than the L-form, but the L-form has a 5-fold higher $k_{cat}$, which one is preferentially metabolized, and how do we decide? 

It was Fersht who first provided the definitive answer, at least for single-substrate enzymes (Fersht, Enzyme Structure and Mechanism, 1977; see also here)


  The kinetic parameter determining the specificity for competing substrates is neither $K_m$ nor $k_{cat}$, but the (so-called) specificity constant: $$\frac{k_{cat}}{K_m}$$


If it assumed that both substrates, considered in isolation, obey the Michaelis-Menten kinetics, then it is very easy to derive (starting with the ratio of the respective Michaelis-Menten equations) the following expression for the initial velocities when both substrates are present simultaneously. 

$$  {
  {{{v_{i}^1}}\over{v_{i}^2}}} =  {{{{k_{cat}^1}}\over{K_{m}^1}[S_1]}}  \times{{{{K_{m}^2}[S_2]}\over{k_{cat}^2}}} \ \ \ \ \ (1)$$

(The equation is derived by, among others, Cornish-Bowden (2004, pp 36-39):

If $V_{max}$ is preferred to $k_{cat}$:
 $$  {
  {{{v_{i}^1}}\over{v_{i}^2}}} =  {{{{V_{max}^1}}\over{K_{m}^1}[S_1]}}  \times{{{{K_{m}^2}[S_2]}\over{V_{max}^2}}} \ \ \ \ \ (2)$$

${{k_{cat}}}\over{K_m}$ has units of a second-order rate constant and, giving it a symbol (${k_{1}^{\prime}}$ for $S_1$ and ${k_{2}^{\prime}}$ for $S_2$) to reflect its status as a fundamental kinetic constant and not just a ratio, allows the following equation to be written: 

$$  {
  {{{v_{i}^1}}\over{v_{i}^2}}} =  {{{{k_{1}^{\prime}}}\over[S_1]}}  \times{{{[S_2]}\over{k_{2}^{\prime}}}} \ \ \ \ \ (3)$$

This equation provides the answer to the OP question. For the racemate described above, we need only calculate the ratio of the specificity constants to get the answer:

$${
    {
     {
      {v_{i}^{D_{form}}
     }
     }\over{
             v_{i}^{L_{form}}}
            }
    }   = 
{
    {
     {
      {k_{D_{form}}^{\prime}\times{[D_{form}]}
     }
     }\over{
             k_{L_{form}}^{\prime}}\times{[L_{form}]}
            }
    } = {
    {
     {
      {k_{D_{form}}^{\prime}
     }
     }\over{
             k_{L_{form}}^{\prime}}
            }
    } =  2 \ \ \ \ \ (4)$$

Much could be written about the specificity constant (diffusion control, enzyme 'perfection', Haldane relationships), but I'll make just one. Considering the single-substrate Michaelis-Menten enzyme, the following holds AT ALL SUBSTRATE CONCENTRATIONS

$$  {
  {{{v_{i}}}}} =  {{{{k_{1}^{\prime}}}}[{S_o}][E]}   \ \ \ \ \ (5)$$

When $S_o$ is very low, $[E]$ = $e_o$ (approx) and the following may be written (where,unlike $[E]$, $e_o$ is usually known or may be determined) 
 $$  {
  {{{v_{i}}}}} =  {{{{k_{1}^{\prime}}}}[{S_o}]e_o}   \ \ \ \ \ (6)$$


  But it is Eqn (5) that is the important one: $k^{\prime}$ (the so-called specificity constant) may be thought of as the second-order rate constant governing the combination of ('free') enzyme with substrate at all substrate concentrations, not just when $S_o$ &lt;&lt; $K_m$ 


Much could be written about this as well

Definitions

I think it is reasonably obvious what everything means. Nevertheless, here is a list: 


$K_{m}^1$  is the Michaelis Constant for substrate 1
$K_{m}^2$  is the Michaelis Constant for substrate 2
$k_{cat}^1 $ is catalytic constant for substrate 1
$k_{cat}^2 $ is the catalytic for substrate 2
$V_{max}^1 $ is the maximum velocity for substrate 1
$V_{max}^2 $ is the maximum velocity for substrate 2
$k_{1}^{\prime} $ is the specificity constant for substrate 1
$k_{2}^{\prime} $ is the specificity constant for substrate 2
$v_{i}^{1} $ is the initial velocity due to substrate 1
$v_{i}^{2} $ is the initial velocity due to substrate 2


Reference


Cornish-Bowden (2004) Fundamentals of Enzyme Kinetics 3rd Edn. Portland Press (London)