In your token expression use an eval statement rather than a <set>. Have a look at how this example dashboard does it.
<dashboard>
<label>split_timechart</label>
<row>
<panel>
<chart>
<search>
<query>| makeresults count=100
| streamstats c
| eval _time=_time - c*10
| eval field1=mvindex(split("F1-Value1,F1-Value2,F1-Value3", ","), random() % 3)
| eval field2=mvindex(split("F2-Value1,F2-Value2,F2-Value3", ","), random() % 3)
| eval combined=field1."_".field2
| timechart fixedrange=f span=15s count by combined</query>
<earliest>-24h@h</earliest>
<latest>now</latest>
<sampleRatio>1</sampleRatio>
</search>
<option name="charting.chart">line</option>
<option name="charting.drilldown">all</option>
<option name="refresh.display">progressbar</option>
<drilldown>
<eval token="field1">mvindex(split($click.name2$,"_"),0)</eval>
<eval token="field2">mvindex(split($click.name2$,"_"),1)</eval>
</drilldown>
</chart>
</panel>
<panel>
<html>
Clicked value was made from a combination of <b style="color:red">$field1$</b> from field 1 and <b style="color:blue">$field2$</b> from field 2
</html>
</panel>
</row>
</dashboard>
