|We want the computers within each company to be able to "see one another" but be blocked from access the computers of the other company.|
Ok you'll excuse me if this doesn't make a whole lot of sense to me.
For what reason on the face of this lovely planet would you want the PC's from the two separate networks to be able to "see each other" but not have access to each other!?!?
If they're not going to access each other, there is no reason for them to see each other............period.
If it were me, I'd separate them completely with a SOHO router for each separate LAN and I'd leave it at that.
Unless of course you have a good reason for a useless "see but can't touch" setup which would involve a whole lot more effort than just segmenting the two apart from each other.