My question is should I allocate another ~140 GB of disk space for BCH full node? Thanks!
If you want to run full BCH node - then yes. Most of these 140Gb is the same for now as with BTC node though, but that's not going to help you in reducing space requirements.
You can always run your node pruned, with ~5-10Gb space requirement max.
If you intend to run both BCH and BTC nodes at the same time on same computer - the issue will be that they both require port 8333 open. Unless you have multiple IPv4 adresses or IPv6 subnet - you will have to choose only 1 node visible to the world.
Yes but you will have to specify different folder for blockchain data because BCH uses the same folder as BTC.
"However, there is a “gotcha”: At least the current Bitmain-coin code will use the original Bitcoin directories by default. This means if you wish to install or use Bitmain-coin yourself, it will mess up your Bitcoin installation! Using both together is outside the scope of this blog post, however, and I personally will only be supporting real Bitcoin." --Luke Dashjr (source)