I held Bitcoin in Multibit before the summer fork and later found that it wasn't supported anymore and installed Electrum. If I install Electroncash and use the same seed phrase, will I recover my Bitcoin Cash and should I also install Electroncash on a seperate machine to Electrum?
If the adress that the seed belongs to held any bitcoin at the time of the split, then yes,
BCH should have been credited to the adress with that seed phrase, since they use the same adress format.
For absolute security, yes, you should. See https://electrum.org/bcc2.txt