Yes, I'm having good success with "Finde englische Serie" as well, but not "Suche englische Serie," which is understandable.
If you choose German phrases that are phonetically in sync with English, I guess it is not bad (e.g. "Find English series" vs. "Finde englische Serie").
Seems like a pretty good compromise.
Technically, then, if you don't want to have to signal the English VC with a special prefix word, you could create a command in your German installation that saves the command recording to the Wav Watch folder of the English version. Then just set up a Wav Watch folder in the "advanced" options tab in the English instance.
Maybe the delay would be not worth it, but it could be a fun experiment. Do you want to try that?
e.g. (If using regular VoxCommando.exe with the German engine)
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.2.0.9-->
<command id="333" name="Finde englische Serie {1}" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>VcAdvanced.SaveRecoWav</cmdType>
<params>
<param>insert filepath to wav watch folder for English configuration of VC.</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Finde englische Serie</phrase>
<payloadDictation>payloadDictation: Regular</payloadDictation>
</command>