Hi - I'm implementing a new tool in Galaxy. The tool either takes 1 or 2 fastq files as input. If 1 file is specified, then 1 output file is created. If 2 inputs are specified, then 2 output files will be created. How do I specify the optional output parameter?

I'm using this as my tool description right now:

<tool id="subsample_fastq" name="Random subsample">
<description>a fastq file</description>
<command>

### Check for optional input file

    #if strlen($fq2) > 0:
      RandomSubFq -w $readsRequested -i "$fq1" -i "$fq2" -o $out1 -o $out2
    #else:
      RandomSubFq -w $readsRequested -i "$fq1" -o $out1
    #end if
</command>
<inputs>
    <param name="fq1" type="data" format="fastq" label="FASTQ file (1st of Pair)"/>
    <param name="fq2" type="data" format="fastq" label="FASTQ file (2nd of Pair)" optional="true"/>
    <param name="readsRequested" type="integer" min="1" value="100000" label="Total number of reads you want from the file"/>
</inputs>
<outputs>
    <data format="fastq" name="out1" />
    <data format="fastq" name="out2" />
</outputs>
<stdio>
    <exit_code range="-1" level="fatal" description="RandomSubFq failed" />
</stdio>