title: Multiple legends in a plot with geom_tile
body: <p>I have two different data frame: P1 and P2. Each of these data frame have 3 different columns: N1, N2 and a value (mean_RMSE). N1 and N2 are between 15 and 120 and for a line, N2 is always lower than N1.</p>
  
  <p>If I plot P1, this is what I get:</p>
  
  <pre><code>p &lt;- ggplot()
  p &lt;- (p
        + geom_tile(data=P1, aes(x=as.numeric(N1), y=as.numeric(N2), fill=mean_RMSE))
  )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/p768L.png" alt="P1"></p>
  
  <p>But my aim is to plot P1 and P2 on the same plot:</p>
  
  <pre><code>p &lt;- ggplot()
  p &lt;- (p
        + geom_tile(data=P1, aes(x=as.numeric(N1), y=as.numeric(N2), fill=mean_RMSE))
        + geom_tile(data=P2, aes(x=as.numeric(N2), y=as.numeric(N1), fill=mean_RMSE))
  )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/I79p6.png" alt="P1_P2"></p>
  
  <p>I don't know how to change the color for P2. For example, I want a blue color range for P1 and a red color range for P2, to easily distinguish P1 from P2.</p>
  
  <p>scale_fill_gradient change the color of P1 and P2 (I still can't distinguish them) and if I add "colour" in geom_tile, I juste have two different contours:</p>
  
  <pre><code>p &lt;- ggplot()
  p &lt;- (p
        + geom_tile(data=psSST_T[[1]], aes(x=as.numeric(N1), y=as.numeric(N2), fill=mean_RMSE, colour="red"))
        + geom_tile(data=psT_SST[[1]], aes(x=as.numeric(N2), y=as.numeric(N1), fill=mean_RMSE, colour="blue")))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Wn4TC.png" alt="contour"></p>
  
  <p>In fact, I want two different legends for P1 and P2.</p>
  
  <p>Can someone help me?</p>